Darksteel Mutation

Bracket

• B1 Exhibition — Not in the same deck as its two-card combo partner: Anzrag, the Quake-Mole
• B2 Core — Not in the same deck as its two-card combo partner: Anzrag, the Quake-Mole
• B3 Upgraded — allowed: Its two-card combos are not early, which Bracket 3 allows
• B4 Optimized — allowed: No restriction beyond the banned list
• B5 cEDH — allowed: No restriction beyond the banned list

Why

• Two-card combo with Anzrag, the Quake-Mole (none of them early)

How Lotus Vault estimates a bracket

Bracket: Lotus Vault analysis, with Commander Spellbook flags

Combos

2 known combos, one of them two-card.

• Anzrag, the Quake-Mole + Darksteel Mutation
  Produces: Infinite combat phases, Infinite untap of creatures you control, Infinite mana creatures you control can produce
  View on Commander Spellbook
• Medomai the Ageless + Capsize + Darksteel Mutation
  Produces: Infinite turns, Lock
  View on Commander Spellbook

Combos: Commander Spellbook

Rulings

• 2013-10-17 · Wizards of the Coast
  The enchanted creature will be only an artifact and a creature, not any other card types. It will be only an Insect, not any other creature types.
• 2013-10-17 · Wizards of the Coast
  In some rare cases, the creature may have subtypes other than creature types before becoming enchanted with Darksteel Mutation. If it had any other artifact subtypes (such as Equipment), it will retain those. If it had any subtypes other than artifact types and creature types (such as Shrine), it won't retain those.
• 2013-10-17 · Wizards of the Coast
  Darksteel Mutation overwrites any previous effects that set the enchanted creature's power or toughness to a specific value. Any such effects that start to apply after Darksteel Mutation entered the battlefield will work normally.
• 2013-10-17 · Wizards of the Coast
  Darksteel Mutation overwrites the printed power and toughness of the enchanted creature, as well as any characteristic-defining abilities that define power and/or toughness.
• 2013-10-17 · Wizards of the Coast
  However, Darksteel Mutation does not overwrite effects that change the enchanted creature's power or toughness without setting it to a specific value (such as the ones created by Giant Growth or Glorious Anthem). It also won't overwrite the effect of counters.
• 2013-10-17 · Wizards of the Coast
  The creature will keep any supertypes it previously had. Notably, if Darksteel Mutation is enchanting a legendary creature, that creature will continue to be legendary. Also, if it's enchanting a commander, that creature will continue to be a commander.
• 2013-10-17 · Wizards of the Coast
  Darksteel Mutation causes the enchanted creature to lose all abilities except indestructible at the time it becomes enchanted. Any abilities the creature gains after that point will work normally.
• 2013-10-17 · Wizards of the Coast
  Darksteel Mutation doesn't affect the enchanted creature's colors, if any. It will continue to be whatever color or colors it was before Darksteel Mutation entered the battlefield.

Rulings: Scryfall · Wizards of the Coast