Nest of Scarabs

Whenever you put one or more -1/-1 counters on a creature, create that many 1/1 black Insect creature tokens.

Amonkhet Remastered

"They are small, but they always want more. Learn from them."
—Bontu, god of ambition

Amonkhet

If a creature with wither or infect deals damage to a creature, the controller of the creature with wither or infect puts that many -1/-1 counters on the second creature.

If an effect has you put more -1/-1 counters on a creature than it has toughness, you’ll put all of those counters on it and create that many Insects, even if that makes its toughness a negative number.