Takklemaggot

Enchant creature
At the beginning of the upkeep of enchanted creature's controller, put a -0/-1 counter on that creature.
When enchanted creature dies, that creature's controller chooses a creature that Takklemaggot could enchant. If the player does, return Takklemaggot to the battlefield under your control attached to that creature. If they don't, return Takklemaggot to the battlefield under your control as a non-Aura enchantment. It loses "enchant creature" and gains "At the beginning of that player's upkeep, Takklemaggot deals 1 damage to that player."

Nebuchadnezzar

{X}, {T}: Choose a card name. Target opponent reveals X cards at random from their hand. Then that player discards all cards with that name revealed this way. Activate only during your turn.

Serpent Generator

{4}, {T}: Create a 1/1 colorless Snake artifact creature token. It has "Whenever this creature deals damage to a player, that player gets a poison counter." (A player with ten or more poison counters loses the game.)

The Fallen

At the beginning of your upkeep, The Fallen deals 1 damage to each opponent and planeswalker it has dealt damage to this game.

Safe Haven

{2}, {T}: Exile target creature you control.
At the beginning of your upkeep, you may sacrifice Safe Haven. If you do, return each card exiled with Safe Haven to the battlefield under its owner's control.

Cromat

{W}{B}: Destroy target creature blocking or blocked by Cromat.
{U}{R}: Cromat gains flying until end of turn.
{B}{G}: Regenerate Cromat.
{R}{W}: Cromat gets +1/+1 until end of turn.
{G}{U}: Put Cromat on top of its owner's library.

Arena of the Ancients

Legendary creatures don't untap during their controllers' untap steps.
When Arena of the Ancients enters, tap all legendary creatures.