Thunderdrum Soloist

Reach
Opus — Whenever you cast an instant or sorcery spell, this creature deals 1 damage to each opponent. If five or more mana was spent to cast that spell, this creature deals 3 damage to each opponent instead.

Zaffai, Thunder Conductor

Magecraft — Whenever you cast or copy an instant or sorcery spell, scry 1. If that spell's mana value is 5 or greater, create a 4/4 blue and red Elemental creature token. If that spell's mana value is 10 or greater, Zaffai deals 10 damage to an opponent chosen at random.

Magma Opus

Magma Opus deals 4 damage divided as you choose among any number of targets. Tap two target permanents. Create a 4/4 blue and red Elemental creature token. Draw two cards.
{U/R}{U/R}, Discard this card: Create a Treasure token.

Expressive Firedancer

Opus — Whenever you cast an instant or sorcery spell, this creature gets +1/+1 until end of turn. If five or more mana was spent to cast that spell, this creature also gains double strike until end of turn.

Improvisation Capstone

Exile cards from the top of your library until you exile cards with total mana value 4 or greater. You may cast any number of spells from among them without paying their mana costs.
Paradigm (Then exile this spell. After you first resolve a spell with this name, you may cast a copy of it from exile without paying its mana cost at the beginning of each of your first main phases.)

Spectacular Skywhale

Flying
Opus — Whenever you cast an instant or sorcery spell, this creature gets +3/+0 until end of turn. If five or more mana was spent to cast that spell, put three +1/+1 counters on this creature instead.

Hostility

Haste
If a spell you control would deal damage to an opponent, prevent that damage. Create a 3/1 red Elemental Shaman creature token with haste for each 1 damage prevented this way.
When Hostility is put into a graveyard from anywhere, shuffle it into its owner's library.

Molten-Core Maestro

Menace
Opus — Whenever you cast an instant or sorcery spell, put a +1/+1 counter on this creature. If five or more mana was spent to cast that spell, add an amount of {R} equal to this creature's power.

Echocasting Symposium

Target player creates a token that's a copy of target creature you control.
Paradigm (Then exile this spell. After you first resolve a spell with this name, you may cast a copy of it from exile without paying its mana cost at the beginning of each of your first main phases.)

Elemental Mascot

Flying, vigilance
Opus — Whenever you cast an instant or sorcery spell, this creature gets +1/+0 until end of turn. If five or more mana was spent to cast that spell, exile the top card of your library. You may play that card until the end of your next turn.

Deluge Virtuoso

When this creature enters, tap target creature an opponent controls and put a stun counter on it. (If a permanent with a stun counter would become untapped, remove one from it instead.)
Opus — Whenever you cast an instant or sorcery spell, this creature gets +1/+1 until end of turn. If five or more mana was spent to cast that spell, this creature gets +2/+2 until end of turn instead.

Eris, Roar of the Storm

This spell costs {2} less to cast for each different mana value among instant and sorcery cards in your graveyard.
Flying, prowess
Whenever you cast your second spell each turn, create a 4/4 red Dragon Elemental creature token with flying and prowess.

Tackle Artist

Trample
Opus — Whenever you cast an instant or sorcery spell, put a +1/+1 counter on this creature. If five or more mana was spent to cast that spell, put two +1/+1 counters on this creature instead.

Exhibition Tidecaller

Opus — Whenever you cast an instant or sorcery spell, target player mills three cards. If five or more mana was spent to cast that spell, that player mills ten cards instead.

Colorstorm Stallion

Ward {1}, haste
Opus — Whenever you cast an instant or sorcery spell, this creature gets +1/+1 until end of turn. If five or more mana was spent to cast that spell, create a token that's a copy of this creature.

Muse Seeker

Opus — Whenever you cast an instant or sorcery spell, draw a card. Then discard a card unless five or more mana was spent to cast that spell.