Whenever you cast an instant or sorcery spell, Muddle becomes a copy of up to one target nonlegendary creature you control until end of turn, except it has myriad. (Whenever it attacks, for each opponent other than defending player, you may create a token copy that's tapped and attacking that player or a planeswalker they control. Exile the tokens at end of combat.)
Cast cheap instants and sorceries to turn Muddle into a copy of your best nonlegendary creature with myriad, then swing wide for big multiplayer damage with temporary token copies. You're an Izzet spellslinger deck that loads the board with strong attackers (or value creatures) and uses spell triggers to convert Muddle into the most impactful body each turn. Win through repeated myriad attacks or by going off with a single broken creature copy.
Myriad makes combat damage scale brutally in multiplayer, hitting every opponent at once
Spellslinger shell gives consistent card flow, interaction, and counterspell protection
Flexible threat assessment: copy whatever creature is best each turn, then reset
Hard to answer the commander since the copy effect is recurring and only lasts a turn
Needs both a good creature on board AND spells in hand to function, so it can stumble
Folds to board wipes that hit your nonlegendary creature targets
Token copies are exiled at end of combat, so no lasting board presence from myriad
Two-card-dependent and can be slow if you draw all spells or all creatures
Generates token bodies that double as myriad copy targets and fuel for go-wide turns.
Tighten the curve with cheaper, higher-impact instants and sorceries (cantrips and one-mana interaction) so Muddle's trigger comes online every turn. Add stronger myriad payoffs—creatures with brutal attack triggers and anthem/deathtouch effects—plus protection like Lightning Greaves and Heroic Intervention. Push toward fast mana (Sol Ring, signets, Jeska's Will) and consistent draw to assemble the creature-plus-spell engine reliably.