Overabundance

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

Manabarbs

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

Doubling Cube

{3}, {T}: Double the amount of each type of unspent mana you have.

Omnath, Locus of Mana

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

Umbral Mantle

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

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}.

Heartbeat of Spring

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

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.)

Bello, Bard of the Brambles

During your turn, each non-Equipment artifact and non-Aura enchantment you control with mana value 4 or greater is a 4/4 Elemental creature in addition to its other types and has indestructible, haste, and "Whenever this creature deals combat damage to a player, draw a card."

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