Urza, Lord High Artificer

Until end of turn, target creature you control becomes a green Wurm with base power and toughness 6/4. Overload 4GG (You may cast this spell for its overload cost. If you do, change "target" in its text to "each.")

(S can be paid with one mana from a snow source.)

If it's not your turn, you may exile a blue card from your hand rather than pay this spell's mana cost. Counter target noncreature spell. If that spell is countered this way, exile it instead of putting it into its owner's graveyard.

Whenever another creature you control enters, put a +1/+1 counter on that creature.

Whenever a creature you control enters, you gain 1 life. If this enchantment isn't a creature, it becomes a 3/3 Angel creature with flying in addition to its other types until end of turn.

You can't spend mana to cast this spell. Convoke, delve (Each creature you tap while casting this spell pays for 1 or one mana of that creature's color. Each card you exile from your graveyard pays for 1.) You may cast this card from your graveyard. Trample

Sliver creatures you control have "When this creature enters, it deals 1 damage to target player or planeswalker and you gain 1 life."

T, Pay 1 life: Add U or R. 1, T, Sacrifice this land: Draw a card.