Whenever you cast a noncreature spell, Accident-Prone Apprentice perpetually gets +1/+1. This ability also triggers if Accident-Prone Apprentice is in exile.

When this creature enters, if you control another Knight, look at the top five cards of your library. You may reveal a Knight, Aura, Equipment, or legendary artifact card from among them and put it into your hand. Put the rest on the bottom of your library in a random order.

: Add one mana of any color.
: Add X mana of any one color, where X is the amount of life you gained this turn.

Equipped creature gets +0/+3 and has vigilance.
Equip

Look at the top three cards of your library. Put one of those cards into your hand and the rest on the bottom of your library in any order. Put each of those cards into your hand instead if there are three or more Lesson cards in your graveyard.

Draw a card, then draw cards equal to the number of cards named Accumulated Knowledge in all graveyards.

When this creature enters, sacrifice a creature.

: Add .
, : Regenerate target Shade, Skeleton, Specter, Spirit, Vampire, Wraith, or Zombie.

: Target attacking Zombie gains indestructible until end of turn.

When this creature enters, each player sacrifices a nontoken creature of their choice.

Spells your opponents cast that target this creature cost less to cast.
When this creature dies, return it to the battlefield transformed under your control attached to target opponent.

Nitro-9 — Whenever Ace attacks, you may sacrifice an artifact. When you do, put a +1/+1 counter on Ace, then it fights up to one target creature defending player controls.
Doctor's companion (You can have two commanders if the other is the Doctor.)

Flying
Whenever you cast a creature spell without flying, conjure a duplicate of it into your hand. The duplicate perpetually gains flying. This ability triggers only once each turn.

Alliance — Whenever another creature you control enters, put a +1/+1 counter on this creature.

Flying
When this creature enters, choose target creature you don't control and tap it. If you control a creature with a counter on it, the chosen creature doesn't untap during its controller's next untap step.

When Acererak enters, if you haven't completed Tomb of Annihilation, return Acererak to its owner's hand and venture into the dungeon.
Whenever Acererak attacks, for each opponent, you create a 2/2 black Zombie creature token unless that player sacrifices a creature of their choice.

Equipped creature gets +3/+0.
As long as equipped creature is attacking, it has first strike and must be blocked by a Dalek if able.
Equip legendary creature
Equip

At the beginning of your upkeep, you may say "Ach! Hans, run! It's the..." and the name of a creature card. If you do, search your library for a card with that name, put it onto the battlefield, then shuffle. That creature gains haste. Exile it at the beginning of the next end step.

Freerunning
Menace
Other Assassins you control get +1/+1.

Destroy all Forests.

Reach
When this creature enters, you may destroy target Equipment.

, : Whenever target creature deals combat damage to a non-Wall creature this turn, destroy that non-Wall creature. When the targeted creature leaves the battlefield this turn, sacrifice this artifact. Activate only before blockers are declared.

Deathtouch
When this creature enters, destroy target artifact, enchantment, or land.

All Slivers have ", Sacrifice this permanent: This permanent deals 2 damage to any target."

Acidic Soil deals damage to each player equal to the number of lands they control.

Flying, deathtouch
Megamorph
When this creature is turned face up, put a +1/+1 counter on each other Dragon creature you control.

When Circle of the Land Druid enters, you may mill four cards.
Natural Recovery — When Circle of the Land Druid dies, return target land card from your graveyard to your hand.