I was unfortunately struck with a somewhat loopy idea today, and I had to write it down. Since I went to all that effort, I figured I'd humiliate myself a little more by posting it here!
There's also a somewhat whimsical backstory (I was trying to get away from the blood-and-thunder stuff that all the other Demigods have) which I will post in segments as replies. The last few abilities are a bit lame -- I was running out of ideas. 
These are some ideas for skills that I've been kicking around in my
head. I tied them together in a Demigod, but I'm mainly interested in
getting some ideas out there other than "ultimately powerful warrior
who does X damage and slows by Y amount for Z time"; the visual
descriptions and backstory are just for fun. He's heavy on skills to
manipulate Mana and on various sorts of channeling abilities, maybe to
an excessive extent. Also: numbers are just general notions, I don't
claim this is balanced or that the allocations of skill points are
right. I deliberately made his spells pricey in terms of mana, for
reasons that should become obvious below.
Really, although I enjoyed putting this together, this is more just a
way of throwing around a bunch of ideas about skills. Feel free to
lift these and use them on another Demigod, or tweak this guy to be
better.
Demigod: Void Blade
Assassin
Void Blade has two forms, his Mortal Form and his True Form.
Mortal form:
Void Blade looks like a human swordsman carrying a REALLY BIG
double-handed weapon. The weapon is dark and covered in shimmering
blue light. Base weapon damage is relatively modest, on the level of
say Oak or Sedna. Mana gained per second reduced by 75% (so if it was
10, now it's 2.5; this is a flat multiplier, applied after any item
buffs)
This is Void Blade's default form, reflecting the long time he has
spent among humans.
===========
Skill 1: Void Strike
NOTES: Void Blade's nuke. Similar to Oak's Penitence in that it is
relatively weak but has a nasty side effect.
DESCRIPTION:
Void Blade channels the energy of the Void for a powerful attack
against his opponent, damaging their reserves of Mana.
Levels 1/4/7/10
0.3 second cast
7 second cooldown
Mana cost: 600/800/1200/1600
Does 200/400/600/800 damage and removes 200/400/600/800 mana.
===========
Level 15: Bloody Severance
Void Blade's Void Strikes also silence the target for three seconds.
===========
Skill 2: World Wound
With a mighty blow of his sword, Void Blade pierces the boundaries of
the world, opening a portal directly to the Void.
(I'm thinking here about the last Earthsea book, where the magic is
draining out a hole in the world, and about a less subtle version of
the Subtle Knife)
(I can't make my mind up whether
the Void lacks mana or is full of it. Maybe there are two Voids, one
low-energy one and one high-energy one? Anyway, I don't care so much
about consistency -- if the skills work someone can make an excuse for them
)
Levels 1/4/7/10
1.0 second cast
45 second cooldown
Mana cost: 1000/1333/1666/2000
Area of effect: 10/12/12/15 yards
Drains 50/100/150/200 mana per second from nearby enemies.
Duration: 10/10/10/10 seconds
At levels 4 and above, affected enemies are sucked towards the center
of the spell. Suction is more powerful at higher levels (but a running
Demigod will always be able to get away -- they'll just be slowed down).
At levels 7 and above, affected enemies get a debuff for the duration
of the spell that doubles the Mana cost of their abilities.
(the reason for only draining enemies is simply that otherwise people
would use this to be griefers)
===========
Level 15: Planar Rent
Void Blade's World Wound damages the borders between the worlds even
further.
Enemies affected by World Wound have the casting time of their spells
increased by 33%.
===========
Skill 3: Mana Storm
Void Blade channels Mana from the Void through his mortal body,
directing it at nearby enemies. While he is channeling this spell, he
may not move, attack, or use abilities. The strain of maintaining
this spell will damage Void Blade's mortal body.
For a visual, he could hold the sword above his head like a cheesy
Final Fantasy move.
Levels 5/10/15
Instant
20 second cooldown
Area of effect: 5/10/20 yards.
Costs 200/200/200 mana per second.
Void Blade takes 100/50/50 damage per second.
Level 5: Nearby enemies are dealt 50 damage per second.
Level 10: The mana storm becomes stronger. Nearby enemies are dealt
75 damage per second and pushed away from Void Blade.
Level 15: Mana Tempest: the mana storm becomes a fearsome cyclone of
arcane force. Nearby enemies are dealt 100 damage per
second and pushed more strongly away from Void Blade.
The force at level 15 should be strong enough that only people who have stacked speed like crazy can get anywhere near him.
===========
Passive Skill: Mortal Acclimation
Void Blade becomes more comfortable in his mortal shell, allowing him
to channel Mana with less of a penalty.
(Players who want to focus on this form would normally buy this skill
to at least one level)
Levels 2/5/8
Mana gained per second is now only reduced by 50/40/20% when Void
Blade is in his mortal form.
===========
Passive Skill: Thirsty Blade
Void Blade's sword gains an aura of negative Voidal energy. His
attacks drain Mana.
Levels 5/10/15
Void Blade's melee strikes remove 5%/7%/10% of the damage dealt from
his target's Mana pool and add it to his own.
True form:
Void Blade looks like a floating, amorphous energy being, perhaps a
bit like a Vorlon without its encounter suit. His attack becomes a
ranged "shooting energy ball thingies" animation that's less
powerful than his attack in his mortal form (say, 50-75% of the damage)
and his innate armor is decreased by say 50%, but his innate movement
speed is now at the same level as the "fast" demigods (UB etc).
===========
Skill 1: Mana Fountain
Void Blade channels Mana directly from the Void. He may not move or
use abilities while this spell is active, and his passive mana and
health regeneration are disabled.
Levels 1/4/7/10
Instant
45 second cooldown
Nearby allies receive 20/40/70/100 mana per second.
Void Blade receives 100 damage per second.
===========
Level 15: Living Fountain
Passive Mana and Health regeneration are no longer disabled while Mana
Fountain is active.
===========
Skill 2: Consume Magic
Void Blade can dismantle magic spells to consume their raw energy.
Healing and damage spells cast on the target heal Void Blade (same
goes for ticks of damage-over-time spells). Void Blade can consume
his own spells.
If cast on an enemy Demigod, this is treated as a debuff (it will be
dispelled by purges or by a Healing Crystal).
Levels 1/4/7/10
0.2 second cast
Mana cost: 500/750/1000/1250 Mana
45 second cooldown
Duration 2/4/6/8 seconds
Void Blade receives health equal to 25/40/55/70% of the damage or
healing that any spell would have performed on the target. The
remainder of the spell is applied to the target as usual.
(alternative: instead, this could trigger only on newly cast spells,
only on the very first spell, and last for 45 seconds)
(griefing potential here -- maybe it should block heals on enemies and
damage on allies and nothing else?)
===========
Skill 3: Cloak of the Void
Void Blade slips halfway into the Void, becoming invisible. While
invisible he cannot attack, move, or use abilities (with the single
exception that he can change to his Mortal Form and back), and his
passive regeneration of health and Mana are disabled. Targeted
abilities will miss him, but he can be affected by area-of-effect
attacks, and items that reveal invisible units will reveal him.
(this would presumably be used to escape people without AoEs until
backup comes, or to pre-emptively cut off the retreat path for enemies
your teammates will attack)
Levels 5/10/15
1.5 second cast
Mana cost: 1000/1250/1500 Mana + 100/75/50 Mana per second.
45 second cooldown
===========
Passive skill: Child of the Void
Level 15
Cloak of the Void no longer disables passive Mana regeneration.
Yes, with some stacking of MPS he can become invisible forever -- but
IMO this is balanced by the fact that he can't do anything while
invisible and he's not actually invulnerable. Alternatively, just
jack the Mana cost up so it's as expensive as Erebus's Mist.
===========
Passive skill: Ethereal Retribution
Melee attacks against Void Blade damage the attacker's Mana.
Levels 5/10/15
5/15/25% of melee damage is applied to the attacker's Mana.
===========
Passive skill: Voidal Hunger
Level 2/5/10
Void Blade devours nearby magic when in his True Form. Nearby enemies
receive a 5/10/15% debuff to their passive Mana regeneration, and any
mana thus drained is given to Void Blade.
Does not stack (drained Mana is split evenly between nearby Blades).
(alternatively: he could get some Health or Mana every time *any*
Demigod *spent* Mana near him)