Desperate Ritual

Add {R}{R}{R}.
Splice onto Arcane {1}{R} (As you cast an Arcane spell, you may reveal this card from your hand and pay its splice cost. If you do, add this card's effects to that spell.)

Narset's Rebuke

Narset's Rebuke deals 5 damage to target creature. Add {U}{R}{W}. If that creature would die this turn, exile it instead.

Brightstone Ritual

Add {R} for each Goblin on the battlefield.

Songs of the Damned

Add {B} for each creature card in your graveyard.

Seething Song

Dragonrage

Add {R} for each attacking creature you control. Until end of turn, attacking creatures you control gain "{R}: This creature gets +1/+0 until end of turn."

Pyretic Ritual

Cabal Ritual

Add {B}{B}{B}.
Threshold — Add {B}{B}{B}{B}{B} instead if seven or more cards are in your graveyard.

Culling the Weak

As an additional cost to cast this spell, sacrifice a creature.
Add {B}{B}{B}{B}.