Once transformed, it becomes Polukranos, Engine of Ruin, which is a 6/6 with reach and lifelink and reads: Whenever Polukranos, Engine of Ruin or another nontoken Hydra you control dies, create a 3/3 green and white Phyrexian Hydra creature token with reach and a 3/3 green and white Phyrexian Hydra creature token with lifelink. Even without any other hydra creatures, the fact this card will split into two is amazing - but with some clever deckbuilding, it could make your deck unkillable
For two mana, you can force an opponent to mill ten precious cards (or at least burn an MTG counter spell to prevent this)
Pot of Greed is probably the most infamous case of this as mentioned below, being a straightforward card that allows the player to draw 2 more cards
Building and updating an EDH deck is an ongoing process
Check out some of them belowits possible you could have an equally valuable card on your hands, too
GR Gold Rare Slifer the Sky Dragon X DIVINE Level 10 [ Divine-BeastEffect ] ATK