This is another one for the "game balance" and "game design" department and it is in regards to the skill mist.
First off, it's a very interesting skill. I like it. I don't want it nerfed or anything like that unless absolutely necessary. However, I encountered the use of it in a game last night on the map Exile that was nearly gamebreaking.
First the scenario:
It was a 4v4 on exile. It was a 46 minute long match (give or take) that was fairly tight up until the last 5 to 10 minutes. Basically, we were all at level 20 or within a few levels of it and both sides had giants going with my side dominating towards the latter part. I was working a Regulus along with our Sedna on a final push. We were held at bay by their Reg and Torchbearer. Between their giants, pults, mines, and flame aoe's it was kind of a stalemate. What I noticed after a while was that our giants never made any forward progress to help us in our push.
So, I took a stroll back to our base and found their Erebus in a PERMANENT mist state at the critical choke point where essentially nothing could pass. The gist of it was that between their reinforcements and his constantly respawning Nightwalkers a "front" was created right over his mist. Our reinforcements would fight there, get the aoe dot, die, and respawn new Nightwalkers.
It was quite ingenious and worked well.
So, I thought, "at 125 man per second, he can't stay in mist forever". Well, I was wrong. When dps'ing down his creep waves (that never seemed to end - partly because his 3 other team-mates were free to handle flag capping and stemming our assault) I thought I'd just camp him for a bit and nail him when he came out of mist. To my surprise he remained mainly in mist the rest of the game. Only leaving it to push forward bit by bit to our citadel. However, his mist "up-time" was well over 95% of his gameplay at this point.
For all it's seeming cheapness, it won the game. Gratz. Kudos to that guy. We lost. Thought I felt like I wasted 46 minutes of my life to 10 minutes of Mist abuse, it wasn't the end of the world.
But it got me thinking - "how the hell did that guy stay in Mist so long?" I've dabbled at Erebus and I know at low levels w/ low mana regen you can't stay in mist long at all.
So I figured I'd test it out and ran a couple of test games where I played the comp and started at level 20 w/ 25,000 gold and high gold regen. I wanted to see what you had to buy to get the high mana regen rates to sustain mist. I found that it wasn't that hard.
Basically it goes like this:
1.) Mist cost is 125 mana per second, while in mist.
2.) Erebus at level 20 (no gear) base mana regen: 7.77 per second (unbuffed)
so approx net cost of mist at this level is 118 mana per second
3.) With these items (all non artifacts) - less than 15K gold (Hungarling, Vlemish, Plenor, Plate visor, and Vinling helm),
112.12 mana per second.
approx net cost of mist is now 13 mana per second and you are sitting at 6884 mana. You can now live in mist for 8 minutes and 48 seconds if you start with a full mana bar.
4.) Drop the el cheapo plate visor and switch to Cloak of Flames (a 9400 gold exchange) and you now have 145.85 mana per second regen plus 7934 total mana.
This will cost you about 23,000 gold total and only 20,300 if you get the price reduction favor item. 23,000 gold by level 20 is not that much as most games people are usually getting 30000 to 40000 gold per game at a minimum, and that's without hitting level 20.
At this point, Erebus can become unkillable at a moment's notice, indefinitely, with a nasty AOE effect. If you camp him, you stand to waste a lot of time. If you don't he just moves on to his next choke point, flag cap, or portal cap, with virtual impunity.
So I gotta ask: Is this intended? 
The only down side of this is that you now have now slots for +health or +speed except for favor slot. However, you can counter both of those w/ a speed wand and heart of life trinket (not to mention never ending bite, bats, and stun).
Now, if this is not intended here are a few fixes I can think of:
1.) With each pulse, the mana cost grows so that after a period of time, it well exceeds any theoretical mana regen state, forcing Erebus to leave mist and open himself up to attack.
2.) Erebus can be "shocked out of mist" after a 20 seconds or so of mist by any DG with a stun ability as well a consumable made available at the shop for DG's that don't have a stun.
3.) Erebus can be targeted in mist if you drop the "see invis" ward.
4.) Cut down or eliminate mana regen in mist state.
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As a side note, I was able to win a game on Cataract vs AI using mist only when pairing it w/ full buffs to Nightwalkers and Vampiric Aura. All I had to do was mist where the enemy's reinforcements tracked and use my Nightwalkers to take down towers. Then all I did was upgrade to priests and camped his citadel in mist. This took down both of his lanes of reinforcements and supplied me w/ never ending Nightwalkers.
I did not put one point into bite, bats, or charm. My only activateable skill was mist. That's it. Granted, this was only vs. Easy AI and we started at level 20 but oddly enough the AI wasn't too bad that game. It never died and actually managed to cap my portals a couple of times (of course I had to recap them). But once I was able to get his reinforcement lanes choked w/ mist, all my reinforcements overwhelmed his base.
This kind of mirrored what happened to me originally on exile.
An interesting way to win, at any rate
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Anyhow, I just wanted to this write-up for several reasons.
1.) Ask the basic questions: Are the devs aware of this? and... Is this working as intended?
2.) If it's working at intended, I'd like to give Erebus players a head's up as to what's achievable with mist and mana regen that's not too hard to obtain in this game. On the flip side, if you see an Erebus player who seems to never have to leave mist, then he probably doesn't have to. You'll have to warn your team mates and adjust to that.
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Finally, if this is working as intended, I have to ask "what kind of game design allows one character out of 8 to remain invulnerable indefinitely?"