The preconstructed Commander deck built around Otrimi, the Ever-Playful gives new players a working shell — Legendary Creature — Nightmare B...
The preconstructed Commander deck built around Otrimi, the Ever-Playful gives new players a working shell — Legendary Creature — Nightmare Beast, colour identity black, green, blue — whose exact text is:
Mutate {1}{B}{G}{U} (If you cast this spell for its mutate cost, put it over or under target non-Human creature you own. They mutate into the creature on top plus all abilities from under it.)
Trample
Whenever this creature deals combat damage to a player, return target creature card with mutate from your graveyard to your hand.
Upgrading a precon is not about replacing it. It is about finding the roles the shell performs weakly — ramp, card draw, interaction, and closing power — and swapping the weakest cards in each role for stronger ones that serve the same job. This guide works by role, which is the honest way to upgrade any prebuilt deck without inventing a decklist you have not verified.
What to cut, by role
Cut cards that duplicate a role you already perform better, cards whose mana cost belongs to a later turn than the role needs, and cards that only function when the game is already going well. Work one role at a time, keep a written list of every swap, and play games between rounds of changes — a precon upgraded on evidence stays coherent, while one rebuilt in a single evening often loses the charm that made it fun.
What to add, by role
Real upgrade candidates that serve this commander's plan include: Boneyard Mycodrax (mana cost {2}{B}; Creature — Fungus; */*; rarity: rare; most recent printing listed by Scryfall: Commander 2020): Boneyard Mycodrax's power and toughness are each equal to the number of other creature cards in your graveyard.
Scavenge {4}{B} ({4}{B}, Exile this card from your graveyard: Put a number of +1/+1 counters equal to this card's power on target creature. Scavenge only as a sorcery.) Gemrazer (mana cost {3}{G}; Creature — Beast; 4/4; rarity: rare; most recent printing listed by Scryfall: Ikoria: Lair of Behemoths): Mutate {1}{G}{G} (If you cast this spell for its mutate cost, put it over or under target non-Human creature you own. They mutate into the creature on top plus all abilities from under it.)
Reach, trample
Whenever this creature mutates, destroy target artifact or enchantment an opponent controls. Brokkos, Apex of Forever (mana cost {2}{B}{G}{U}; Legendary Creature — Nightmare Beast Elemental; 6/6; rarity: mythic; most recent printing listed by Scryfall: Ikoria: Lair of Behemoths): Mutate {2}{U/B}{G}{G} (If you cast this spell for its mutate cost, put it over or under target non-Human creature you own. They mutate into the creature on top plus all abilities from under it.)
Trample
You may cast this card from your graveyard using its mutate ability.
Build and check your list with the MTGBrew AI deck builder, and browse every card mentioned here in the MTGBrew card database on MTGBrew.
If this topic is on your bench, keep going with Rules: What Does 'Legendary' Mean for Your Commander and the Legend Rule? — it covers the neighbouring decisions from a different angle, and the two pieces together give you a fuller picture than either one alone.
Keep reading
Keep reading: Rules: What Does 'Legendary' Mean for Your Commander and the Legend Rule? and Ragavan, Nimble Pilferer in Modern: One-Drop Pressure and Treasure.
A sane upgrade order
Tune in small steps. Change two or three cards at a time, play several games, and write down what actually happened rather than what you hoped would happen. A list adjusted on evidence converges on its best version; a list rebuilt after every loss never stays still long enough to be judged.
Start with the mana base and the draw package, then interaction, then finishers. That order feels unglamorous and wins the most games, because a deck that always makes its land drops and always has cards in hand gets to show you what its exciting cards can actually do.
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