Sacrificial Altar seems to be a rarely used (in my experience) card with a tremendous amount of potential.
For 4 mana, the Altar (zone exclusive conjuration, Dark Mage Only) allows you to do the following:
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Once per round, before or after a friendly activation, you may destroy a creature you own and control to grant another creature +X Melee and +X Piercing until the end of the round.
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Now the first time I saw that card, I thought "This is terrible." Sacrificing a level-3 creature is like paying 17 mana (13 for the creature and 4 for the Altar) to give 3 dice and 3 piercing to a single melee strike! However, on second thought I am convinced that the Altar is incredibly awesome.
1) Sacrifice creatures that would have died anyways.
Got a Darkfenne Bat with 1hp and an Idol of Pestilence out? Make one last attack with the bat, and at the end of his activation Sacrifice him to buff one of your other creatures. It doesn't cost any mana or any actions!
Same goes for those situations where your opponent is focus-firing down one of your bigger creatures. That flaming hellion with 2hp left isn't going to last very long with 2 Knights of Westlock beating on it. So make its attack and then Sacrifice it at the end of the attack action.
For 1 quick action and 4 mana, if you can sacrifice a single level-2+ creature "for free", the Altar has paid for itself.
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2) Sacrifice works well with Battle Fury and/or Retaliate.
Unlike temporary bonuses that are worded as +X until the next attack, Sacrificial Altar gives +X/+X until the end of the round.
+3 Melee and +3 Piercing in exchange for a level-3 creature doesn't sound great... but +3 Melee and +3 Piercing for two (or three) attacks in one round is amazing.
It is theoretically possible to roll 30 attack dice with +6 Piercing: Take a Warlock with Lash, Bear Strength, Gaunts and Fireshaper Ring (9 dice total), sacrifice a Lord of Fire (+6 = 15 dice) and perform a Battle Fury attack (15 dice + 15 dice). Granted, it's a super expensive combo, but it could easily kill someone in one round (hope they didn't have Reverse Attack!).
As an extra bonus, if your warlock was hiding a Retaliate on him, he could retaliate an attack for another 15 dice. That's 45 attack dice (with +6 Piercing) in one round!
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3) Sacrifice completely ignores range, line of sight, incapacitation and banishment.
Did your opponent trap your Flaming Hellion in a corner square with 2 stone walls? Did he Sleep your Vampiress out of range of anything that can wake her? Did he just Banish your Lord of Fire?
Sacrificial Altar gives you an extra option. You can try to break the control spells and keep your creature in play, or you can Sacrifice the creature and Battle Fury for mega damage.
The mere possibility that you could Sacrifice a crowd controlled "big" and insta-gib your opponent can force him to play more cautiously. Or if he doesn't, you can make him eat 30 dice with +6 piercing.
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4) Sacrifice can "force" your opponent to waste attack dice on over-kill.
Your Lord of Fire has 2hp remaining and is stuck in a Force Crush. Sucks for you. Normally your opponent would ignore it and allow it to die its inevitable death.
However, you have a Warlock, a Bat and an Imp all in position to attack your opponent, and you could Sacrifice the Firelord to buff any one of the three. So your opponent uses his quickcast to throw an Invisible Fist and finish off the Firelord.
That's 4 mana and 1 action that he no longer has.