Sekolah is the Power of the sahuagin, the so-called sea devils of the Prime's oceans. He appears as a huge shark and his hunger is neverending. His followers are thusly ever sacrificing sentients on their altars to him. His Realm, Sheyruushk, is located in the dark waters of Baator's Layer of Stygia.
The sahuagin petitioners in Shreyruushk surround Sekolah in perfect geometric formations, and any who falls out of line are eaten. Sekolah is a craft, evil beast, and has been known to surface to converse with (and devour) Baatezu ambassadors to the monarch of the deep.
Long ago, Sekolah enslaved Demogorgon using a powerful artifact. Although he has since escaped from this hold, the Prince of Demons has sworn revenge.
Source: Monster Mythology, Planes of Law, A Paladin in Hell, Manual of the Planes (1st edition), Fiendish Codex II, http://www.wizards.com/rpga/downloads/LG_Deities.zip