The god of pain that Maggie's Celestial Grenade would have summoned to torturously slaughter Carl and the town.
Algos is the god of pain, the deity Maggie My was armed to worship and summon via her Celestial Grenade to kill Carl and everyone in Hump Town. The Skull Empire gave Maggie a scroll letting her choose a god and a box picturing Algos so she'd know whom to pick — a workaround, Mordecai reasons, because her Infiltrator race apparently can't worship Grull, and because the princes didn't want to rely again on the bungling Prince Maestro. Algos is, per Mordecai, "a good god to choose if you want your target to suffer" — he would have killed everyone in town in seconds, in agony.
His summoning is averted when Carl pockets the grenade.
A deity of pain; unseen (never summoned), known only by the box-image the Skull Empire used to identify him to Maggie.
The pain-god weaponized (unsuccessfully) against Carl — an averted instrument of mass torment.