This is my current Sedna build.  It sacrifices early pounce levels and demigod killing power for healing and HPS.  The build is designed to provide excellent support, hold flags and draw attacks and mana from the opponent. You will likely win games based on war score alone by denying flags.  As with any Sedna build, if you have bad teammates it won't matter what you do.  You will only be a serious threat to other demigods after level 10.

Favor Item:  Blood of the Fallen

Skills:
Level 1:  Heal 1
Level 2:  Healing Wind 1
Level 3:  Pounce 1
Level 4:  Heal 2
Level 5:  Healing Wing 2
Level 6:  Inner Grace 1
Level 7:  Heal 3
Level 8:  Inner Grace 2
Level 9:  Counter Heal/Inner Grace 3 (depending on whether the opponent has Priests out)
Level 10: Heal 4
Level 11:  Pounce 2 
Level 12:  Pounce 3
Level 13:  Pounce 4
Level 14:  Inner Grace 3 (assuming you took Counter Heal at 9)
Level 15-20  Magnificent Presence, Silence or Life's Child as suits you, but the game should be over by now anyhow.

* If you do not take Inner Grace 3 at 9, you can take it at any point afterwards as you see fit.  As soon as enemy Priests come out, you need to take Counter Heal.

Equipment

Start:  Monk Idol
2000:  Unbreakable Boots + Scale Mail
1800:  Currency 1
1750:  Vlemish
1500:  Nimoth Chest Armor
2000:  Sell Monks buy Bishops
1750:  Hauberk of Life
5000:  Sell Banded, buy Grofling

Depending on your war score and the situation, buy pots, scrolls and locks as needed

Strategy:
Early game (Pre-Unbreakable).  You have enough mana for two heals.  You will want to fight head-to-head for flags, knowing that nobody else has a whole lot of mana.  Soak up attacks and then retreat back to your tower.  Your monks will keep you in the fight, so if you get a bit low on health just wait at your tower as they heal you.  Try to hold off on your heal as long as possible in order to keep your opponent in the fight.  You want to use your heal only when necessary so that you have mana when you need it.  Maintain your flags and farm creeps.  Avoid all double teams.  Monks are overly aggressive, so make sure to micro them behind you if you get too close to an enemy tower.  This takes some management.  If you are in your lane alone, order your monks to stand by the flag and attack the tower solo.  Retreat to your monks for healing when still above 50%.  Wash, rinse, repeat. 

IMPORTANT:  If you have an assasin in the other lane without general support, you must send a monk to heal him/her.  Do this as soon as possible.  Splitting monks is critical in the beginning of the game as a support monk can keep your teammate in their lane if they are up against anyone without monk support.  Left click on one of your monks and then right-click on your teammate.  That's all you have to do and it will win win win you games.

Early game (Post-Unbreakable):  Getting currency as quickly as possible will make things easier later in the game for buying creeps and better armor.  You will have a bit of speed now, thanks to Inner Grace and you will be able to run past the first towers without much problem.  Keep an eye for anyone looking squishy or slow.  They will often retreat to their tower assuming you will break off.  You can now pursue past towers which can net some kills on the unaware.  Be extremely careful, though, because you still do not have armor.  An enemy DG coming in behind you while pursuing, teleporting in, or a sniper anywhere on the map will ruin your day.  Keep your monks on the field healing your teammates when you return to upgrade.  There's no point in having them with you at the shop.  Use Pounce 1 very rarely, as you don't have Vlemish yet and your mana is better saved for healing.  There are three scenarios to use early Pounce though:

1.  Guaranteed kill.  You're chasing someone an their HP drops below 400.  Kinda obvious.
2.  Burst piling.  If you're in a 2v2 or 2v1 scenario and your teammate is going to throw burst damage or a stun, tack on an extra 400 points with a pounce (fireballs, slams, pentinence, bite, etc)  Pounce I alone isn't going to kill anyone, but combined with a teammate, the damage, interrupt and mini-stun it provides can get some kills
3.  Interrupt.  Again, obvious.  Stop slams, snipes, heals, teleports, whatever.  Do this only when it will make a difference though.  If a Reg is going to snipe you point blank and you have full health, just suck it up and save the mana. 

Use pounce as rarely as possible at this point.  It's better for your enemies to forget you have it and also you just don't have the mana to spam it.  Pounce away all your mana and you will not be able to heal when you need to.

Once you have UB boots, you can solo an undefended tower fairly easily.

Mid game (Pre-Bishops):  You now have good armor, health and very high HPS.  You can also heal 1200 health.  You're now a hard kill.  Your damage is still low.  You still will want to dominate flags and lanes.  Concentrate on pushing back enemy DGs or forcing them to spam their mana on you.  As you become more confrontational, the biggest threat to you is a stun.  Although it happens rarely, getting stunned out of a heal is usually a death sentence.  If an enemy is particularly good at stunning you out of heals, use Pounce to cover your heal.  Concentrate on the meta-game and protecting your teammates.  Grinding down towers is a priority now.

You must have at least one TP scroll in your inventory at all times now.  Preferably more.  Keep aware of your teammates and teleport into fights.  Your presence will instanly unbalance any fight as you start throwing out heals.  Just teleporting in with your Bishops will usually cause them to throw out their heals on your teammates.  If you see a teammate fleeing, tell them to flee towards you so you can heal them.  There's nothing more frustrating than a teammate outrunning your healing.

It is very important that you do NOT get distracted from farming creeps at this point.  It is very easy to get too focused on supporting and tanking.  If you do, you will fall behind in levels and it is VERY hard for Sedna to make that up because she farms so poorly.  Don't get stuck fighting over flags without creeps going past for very long.  Another DG with AOE can make up that experience quickly but you cannot.

Mid game (Post-Bishops):  At this point, you're a formidable tank.  You should have over 5,000 HP and the addition of Bishops will give you free 1000+ HP heals.  Monks are no longer survivable at this level and you'll need to upgrade.  You should be able to pursue past more than one layer of towers with ease. You can be more aggressive with pounce, but still do NOT pounce whenever it is available.  You will run out of mana.  If your enemy concentrates on you in a 2+v2+, retreat to draw them away from their towers while your teammate deals damage.  Your heals and bishops will keep you alive while your teammate gets in free hits.  If they break off, turn on them aggressively and pursue, as your bishops will heal you back up very quickly.  If they focus on your teammate, autoattack your teammate's target and use heal on your teammate.  Between your active heals and you bishops you'll be pushing out 2000+ health to your teammate every few seconds.

Late game:  You're almost impossible to kill at this point.  You're healing over 100 HPS, you have great armor and stacked health.  You will go from Pounce I to a 1000 damage pounce in 3 levels.  Opponents who were used to slugging it out with you will now find you're much more dangerous.  They will tend to flee too late, and then get caught by your pounce.  Tower damage is trivial and you can sail into their backfield capping flags and portals or finishing off kills.  You should have a War Score lead and cap-locking their portals will end things quickly.

Your biggest weakness will become apparent now though.  You have slow attack speed and no AOE to speak of.  Once cats and then giants come into play you will find you do not have the power to fight back creep waves.  If the other team has a War Score lead, you're going to be swarmed under.  Nature's Reckoning can help, although at a cost of some stacked health.  NR does 250 points AOE and heal IV does 200 points AOE.  This is enough to kill everything in a wave except Giants and Angels, assuming no citadel upgrades.  If you see NR proc, you can lay down a heal and hopefull get a lot of kills on little creeps.  You really don't want the game to get to this point, though.  But if you're behind by a little in war score and your opponent grabs cats fast, this can help.  It can also help you catch up in experience if you fell behind.

What has changed from the last Panther build? 
1.  Removed Narmoth's ring, replaced with Hauberk of Life.  You attack too slowly for the health drain of the ring to be effective and you get most of the benefit of the ring from the Hauberk for less than half the cost.  It also makes your Bishops more durable. 
2.  Pounce I taken at level 3 for reasons explained above.
3.  UB Boots and Scalemail taken instead of Vlemish as initial purchase.  Relying on Monks more heavily for heals means you need less mana up front.  The boots give you plenty for the early levels plus HP for less money.  Throw on the scalemail and you have a 1000 HP stack, making your monks that much more effective
4.  Monks upgraded to Bishops during the midgame.

Sila is a Leopard.  Why is this called the Panther build?  It's named after the Panther Tank.  The Panther had probably the best mix of mobility and firepower with maximized armor of any tank in WWII.  Fortunately for Sedna, Sila does not have the maintenance problems that the Panther had.

Why take monks?  I hated monks because they never healed when I wanted or needed them.  And monks are still not good in an aggressive build.  An aggressive monk-free Sedna is good when you have great teammates, but in pub games, I found I would be punished for being aggressive too often due to lack of support.  Using monks early opens up some new strategies.  First you can take on towers immediately when you're alone.  You will not need pots or heal (i.e. mana) to get back to strength, which means you can retreat to the flag you're holding and wait for your monks to heal you back up.  Second, they help in getting you money because their added damage makes farming easier.  Third, they support your teammates.  You can leave them on the battlefield whenever you need to go into town.  And if you split your monks during the early game, you will always beat the team that didn't split.

Pathing still sucks.

Why not take more pounce earlier?  Simple:  mana.  Pounce does less damage per mana than heal recovers health.  The build focuses on always having heal available.  You do not have the DPS to seriously threaten many DGs unless you've been locked in melee for some time.  So you don't really have much chance to get DG kills at low level anyhow.  The benefits of skipping pounce are better healing and faster speed, both which will keep you alive at lower levels.  The interrupt of pounce is too important to skip, though.  Therefore you need to take the first level of pounce early.

What about silence?  Silence has a short duration and takes a ton of mana.  Builds centered around silence require much more mana than this one.  This build is sacraficing mana for health and HPS.  The typical silence use would be combined with pounce or supporting a gank.  While these are legit strategies, they're not used in this build.  Hey, you can't do everything.

How will I die? And how do I prevent it?  You'll likely die for the following reasons:
1.  You were stupid.  You got your health and armor up and decided you were invincible.  You ran after that Reg with 1,000 health left and find yourself surrounded in the middle of the enemy's towers.  Solution:  Don't be stupid.  Play zoomed out enough to see what's going on in other lanes and retreat early and often.  As a rule of thumb, you never want an enemy flag behind you.  This provides a teleport target for the enemy.  Similarly, if you see an enemy pursue past one of your flags, this is a great ganking opportunity for you and your team.  Always keep your lines to your rear open and get out of exposed positions immediately.
2.  Your teammates were stupid.  Your teammate is fighting and you and an opponent rushes to help.  You throw down your heal on him (or her) and they run away anyhow.  You're then stun-ganked while waiting for your heal to cool down.  You bait two 1/3 HP enemies into one of your fresh teammates.  Your teammate farms creeps instead of helping you out.  Etc.  The ways to die from stupid teammates is almost infinite.  Because you are always risking your own safety by healing teammates and you rely on them for higher DPS attacks, Sedna will always have this weakness.  If you can't handle this, I suggest UB, Reg, TB or tower Rook.  It is important to communicate to your teammates, to have them flee towards you so you can heal them or otherwise stand their ground.
3.  You were stun/slow-ganked.  There are some nasty combos out there that will kill you before you can heal.  UB foul grasp - Ere swarms across you - Ere charms/bites is particularly nasty.  Reg Shrapnel mines - TB fireball - Reg snipe.  Posion dagger while you're in a Rook tower farm.  Solution:  Rember that level 3 heal clears all status effects.  This will wipe out slow effects including poison dagger.  Heal them off even if you're at full health.  Most of these situations can be avoided by remaining aware.  But remember you are a target, especially early on.  The enemy will not be afraid of your DPS but will want to remove your heals.  If you're getting swarmed under by speedier builds determined to kill you, Sigils or a Wand of Speed can be bought to improve your escaping power.  Normally, these aren't necessary, but remain flexible.