Lore Seeker

When Bull Elephant enters the battlefield, sacrifice it unless you return two Forests you control to their owner's hand.

Enchant creature
Enchanted creature gets +1/+1.
Sacrifice a creature: Regenerate enchanted creature.

Flash

{3}{B}{B}, Sacrifice a creature: Return Necrosavant from your graveyard to the battlefield. Activate this ability only during your upkeep.

When Ovinomancer enters the battlefield, sacrifice it unless you return three basic lands you control to their owner's hand.
{T}, Return Ovinomancer to its owner's hand: Destroy target creature. It can't be regenerated. That creature's controller creates a 0/1 green Sheep creature token.

This turn and next turn, creatures can't attack, and players and permanents can't be the targets of spells or activated abilities.

{B}, Sacrifice Urborg Mindsucker: Target opponent discards a card at random. Activate this ability only any time you could cast a sorcery.

Enchant creature
When Vampirism enters the battlefield, draw a card at the beginning of the next turn's upkeep.
Enchanted creature gets +1/+1 for each other creature you control.
Other creatures you control get -1/-1.

Haste
At the beginning of the end step, return Viashino Sandstalker to its owner's hand.

As an additional cost to cast this spell, sacrifice a creature.
Target creature gets +4/+2 until end of turn.