Essence Reliquary is a really powerful blink engine, that has many micro synergies in this cube. It can:

• reuse your ETB effects
• bounce your cycling cards (to cycle them away)
• provide more cards to loot away
• returns your madness cards (so you can discard them again for value)
• protect your creatures during your turn
• and much more

The only thing i dislike about this card is that it will probably overshadow my pet card astral drift, but luckily both cards will also work very well together.

Purify the Grave was unnecessary graveyard hate and very rarely played.