Overabundance

Whenever a player taps a land for mana, that player adds one mana of any type that land produced, and this enchantment deals 1 damage to the player.

Harsh Mentor

Whenever an opponent activates an ability of an artifact, creature, or land on the battlefield, if it isn't a mana ability, this creature deals 2 damage to that player.

Umbral Mantle

Equipped creature has "{3}, {Q}: This creature gets +2/+2 until end of turn." ({Q} is the untap symbol.)
Equip {0}

Goblin Anarchomancer

Each spell you cast that's red or green costs {1} less to cast.

Seedborn Muse

Untap all permanents you control during each other player's untap step.

Magus of the Vineyard

At the beginning of each player's first main phase, that player adds {G}{G}.

Citadel of Pain

At the beginning of each player's end step, this enchantment deals X damage to that player, where X is the number of untapped lands they control.

Mana Flare

Whenever a player taps a land for mana, that player adds one mana of any type that land produced.

Ignoble Hierarch

Exalted (Whenever a creature you control attacks alone, that creature gets +1/+1 until end of turn.)
{T}: Add {B}, {R}, or {G}.

Arcane Signet

{T}: Add one mana of any color in your commander's color identity.

Staff of Domination

{1}: Untap this artifact.
{2}, {T}: You gain 1 life.
{3}, {T}: Untap target creature.
{4}, {T}: Tap target creature.
{5}, {T}: Draw a card.

Rug of Smothering

Flying
Whenever a player casts a spell, they lose 1 life for each spell they've cast this turn.

Belbe, Corrupted Observer

At the beginning of each postcombat main phase, the active player adds {C}{C} for each of your opponents who lost life this turn. (Damage causes loss of life.)

Exsanguinate

Each opponent loses X life. You gain life equal to the life lost this way.

War's Toll

Whenever an opponent taps a land for mana, tap all lands that player controls.
If a creature an opponent controls attacks, all creatures that opponent controls attack if able.

Omnath, Locus of Mana

You don't lose unspent green mana as steps and phases end.
Omnath gets +1/+1 for each unspent green mana you have.

Manabarbs

Whenever a player taps a land for mana, this enchantment deals 1 damage to that player.

Sol Ring

Zhur-Taa Ancient

Horizon Stone

If you would lose unspent mana, that mana becomes colorless instead.

Eladamri's Vineyard

Dictate of Karametra

Flash
Whenever a player taps a land for mana, that player adds one mana of any type that land produced.

Yurlok of Scorch Thrash

Vigilance
A player losing unspent mana causes that player to lose that much life.
{1}, {T}: Each player adds {B}{R}{G}.

Kaervek the Merciless

Whenever an opponent casts a spell, Kaervek deals damage equal to that spell's mana value to any target.

Heartbeat of Spring

Scytheclaw Raptor

Whenever a player casts a spell, if it's not their turn, this creature deals 4 damage to them.

Magus of the Candelabra

Leyline Tyrant

Flying
You don't lose unspent red mana as steps and phases end.
When this creature dies, you may pay any amount of {R}. When you do, it deals that much damage to any target.

Ruric Thar, the Unbowed

Vigilance, reach
Ruric Thar attacks each combat if able.
Whenever a player casts a noncreature spell, Ruric Thar deals 6 damage to that player.