Here are my ideas so far for Elder Beings. They are not really complete or balanced at this time (I feel particularly bad about the terrible names); but, I figure it's a decent start and may get others brainstorming so...
Malefic Magebane - Cost: 30 mana (plus 2 Sacrifice tokens), Upkeep 3, Attack 5 with Mana Drain +2, Armor 3, Health 20, Regen 2.
Malefic Magebane is a powerful being who's presence disrupts the flow of magical energy in the world. ALL spells have their cost increased by 2. All spells targeting Malefic Magebane have their cost doubled. No spells can be cast in the same zone as Malefic Magebane.
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Tentacled Horror - Cost: 30 mana (plus 1 Sacrifice Tokens), Upkeep 4, Defense 11+ (infinite), Slow. Regen 4. Armor 3, Health 25.
Quick Attack 4 (reach), Full attack 5 plus d12 to see if target is Incapacitated.
Tentacled Horror has an automatic melee attack against all creatures within 1 zone radius during the upkeep phase. Whenever a creature is killed by Tentacled Horror it gains a +1 Attack token.
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Goddess of Dark Beauty - Cost: 35 Mana (plus 3 Sacrifice Tokens), Upkeep 2, Regen 2, Armor 2, Health 20
All living creatures in zone with Goddess of Dark Beauty must roll 10+ or be incapacitated.
Quick Attack - 4, vampiric
Full Action - Goddess of Dark Beauty takes control of 1 incapacitated creature. That creature can and will attack it's friends and allies if directed to. Every controlled creature gains the Upkeep +2 trait and you maintain control of them so long as that upkeep is paid.
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Brutus Bloodlust - Cost: 30 Mana (plus 1 Sacrifice Token), Upkeep 3, Regen 3, Armor 4, Health 30
Brutus Bloodlust has the Fast and Charge +2 traits.
All Dark Living Creatures gain Bloodlust +2, Armor +1 and Life +3 so long as Brutus Bloodlust is in play.
Full Action - Sweeping attack for 5.
Quick Action - Attack for 6
I rather like the idea that if someone is playing the Voidbringer you have no idea which of the 4 (or more) Elder Beings they are planning on summoning. All the costs and stats are just a rough estimate, playtesting will be needed to keep these at all balanced (as much as summoned Elder Beings can be balanced.)
The real idea is that opposing players need to destroy the Elder Gate before they are summoned and the most powerful take the longest to summon. So long as they are difficult enough to summon they shouldn't be gamebreaking and I rather think that the new dynamic of attack/defend the Elder Gate would make for an interesting game.