, Sacrifice a Rat: Create X 1/1 black Rat creature tokens, where X is the number of Rats you control.
Marrow-Gnawer is a mono-black Rat tribal commander that snowballs token counts exponentially: sacrifice one Rat to make a number of tokens equal to how many Rats you already control. You build a wide board of Rats, activate the ability to double-plus your army, give them all evasion via Marrow-Gnawer's fear, and swing for lethal—often aided by an anthem or overrun effect. Early turns ramp and deploy cheap Rats; mid-game you snowball with the tap ability and protect the board.
Exponential token generation that can produce a lethal army in a single turn once you have a few Rats
Built-in evasion (fear) makes the whole team nearly unblockable in most metas
Mono-black gives access to powerful ramp, tutors, and recursion
Wide boards synergize with aristocrats payoffs and overrun-style finishers
Extremely vulnerable to board wipes; one Wrath can undo all your work
The combo relies on Marrow-Gnawer surviving and being untapped, so removal and tap-down effects hurt
Tribal Rats are a shallow card pool with many weak bodies
Summoning sickness and tap requirement mean it telegraphs the turn before it goes off
Fear doesn't stop artifact or black creatures from blocking
While not a Rat himself in flavor he supports tokens, but more importantly mono-black token doublers like this amplify your output (use a true doubler).
Legendary Creature — Rat Rogue
, Sacrifice a Rat: Create X 1/1 black Rat creature tokens, where X is the number of Rats you control.