The Serioli were the original native inhabitants of Dragaera. They were largely wiped out by the early Dragaeran Empire, and are now living in small numbers in the mountains. The Serioli are humanoid, but smaller than both Dragaerans and Easterners.
The Serioli are the creators of all Morganti weapons, including the Great Weapons. Legends holds they developed the technique to create Morganti weapons in order to make war so terrible no one would ever engage in one. It allegedly worked - among the Serioli.
However, Sethra Lavode (who ought to know) has stated that the Serioli specifically created the Great Weapons to use against the Gods, since the Gods were supporting the Dragaerans who were invading and taking over the Serioli lands.
In response, some factions of the Serioli have, from time to time, aided the Jenoine in their attacks against the Gods, and the holders of the Great Weapons. This is despite the fact that if the Jenoine returned to Dragaera, the Serioli would likely be enslaved along with everyone else.
In Dragon, Vlad Taltos and Morrolan e'Drien traveled to Hawk Mountain in the Kanefthali mountain chain in order to meet with a Serioli. His name was not recorded alphabetically but sounded like The Last Cough from a Man with Juiner's Lung, according to Vlad. This Serioli proceeded to disclose to Vlad and Morrolan certain information about the Great Weapons, which included translations of their Serioli names, and also giving Vlad a hint about Spellbreaker. He also made it clear that the Great Weapons were, to him, guests of the same order as the Dragaeran, the Easterner, and the jhereg — although one was "not altogether here".
Apparently the language of sorcery (the runes and such) is the Seroli written language, and they are skilled at smithing things—their silver products such as jewelry or buttons are highly prized. We've also heard of bone dancers, whatever that may be.
They also make fine dessert wines.