The twin giants who tried to pile Mount Pelion on Mount Ossa to reach Olympus were the Aloadae.
The Aloadae were the brothers Otus and Ephialtes, gigantic sons associated with Aloeus. In their assault on the Olympian gods, they planned to stack mountains so they could climb to heaven. Ancient versions differ over the exact order: Mount Ossa and Mount Pelion are variously placed atop one another, with Olympus as the target below.
Their ambitions went beyond mountain-building. The brothers intended to challenge the gods and, in some accounts, claim Artemis and Hera as wives. They also captured Ares and confined him in a bronze storage jar for thirteen months before Hermes rescued him.
The Aloadae eventually died through a trick involving Artemis. She changed into a deer and ran between them; each brother threw his spear at the animal and accidentally killed the other. They are sometimes confused with the Gigantes, another group of giant enemies of the Olympians, but the mountain-piling episode belongs specifically to Otus and Ephialtes.