Kronis was one of Skeletor's original evil warriors.

Following a dispute of leadership, Kronis was evicted from Snake Mountain and left to fend for himseld against the wilds of the Dark Hemisphere of Eternia.

Swearing revenge on Skeletor, Kronis united the barbarian armies of the Dark Hemisphere.
His plan was to over throw Skeletor and rule in his place.

He failed.

Kronis was badly injured in battle and lost his arm, jaw and had his face burnt off. (See the TPB ICONS OF EVIL for the gory details)

Skeletor had Tri-Klops rebuild Kronis into Trap-Jaw.