You may have this creature enter as a copy of any creature on the battlefield, except it isn't legendary, is an artifact in addition to its other types, and has myriad. (Whenever it attacks, for each opponent other than defending player, you may create a token copy that's tapped and attacking that player or a planeswalker they control. Exile the tokens at end of combat.)

Flying
At the beginning of your upkeep, sacrifice this creature unless you pay .

Each instant and sorcery card in your graveyard gains flashback until end of turn. The flashback cost is equal to its mana cost.
Flashback (You may cast this card from your graveyard for its flashback cost. Then exile it.)

If you would copy a spell one or more times, instead copy it that many times plus an additional time. You may choose new targets for the additional copy.
, : Copy target instant or sorcery spell you control. You may choose new targets for the copy.

Your opponents can't cast spells from anywhere other than their hands.

Exile this card from your hand: Add .

: Add .
, Sacrifice this land: Destroy target nonbasic land.

Target player mills three cards.
Storm (When you cast this spell, copy it for each spell cast before it this turn. You may choose new targets for the copies.)

Haste
Whenever a creature an opponent controls dies, put a +1/+1 counter on Yahenni.
Sacrifice another creature: Yahenni gains indestructible until end of turn.

Whenever an opponent draws a card, this enchantment deals 1 damage to that player.

(: Add .)

Double target player's life total. Shuffle Beacon of Immortality into its owner's library.

Zombie spells you cast cost less to cast.
Zombie creatures you control get +2/+1.

(: Add , , or .)
This land enters tapped.
Cycling (, Discard this card: Draw a card.)

When Urza enters, create a 0/0 colorless Construct artifact creature token with "This token gets +1/+1 for each artifact you control."
Tap an untapped artifact you control: Add .
: Shuffle your library, then exile the top card. Until end of turn, you may play that card without paying its mana cost.

(Gain the next level as a sorcery to add its ability.)
Whenever a creature you control deals combat damage to a player, exile the top card of that player's library face down. You may look at it for as long as it remains exiled.
: Level 2
Creatures you control have menace.
: Level 3
You may play cards exiled with this Class, and you may spend mana as though it were mana of any color to cast those spells.

As an additional cost to cast this spell, sacrifice a creature.
Search your library for a card, put that card into your hand, then shuffle.

Tap another untapped creature you control: Tyvar gains indestructible until end of turn. Tap it.
: Creatures you control get +X/+X until end of turn, where X is the greatest power among creatures you control.

( can be paid with either or 2 life.)
, , Sacrifice a creature: Search your library for a creature card with mana value equal to 1 plus the sacrificed creature's mana value, put that card onto the battlefield, then shuffle. Activate only as a sorcery.

Skeletons you control and other Zombies you control get +1/+1 and have deathtouch. (Any amount of damage they deal to a creature is enough to destroy it.)