Nial Declann is a human Force-sensitive who fought for the Republic in the Clone Wars and served the Empire both as one of its twelve Grand Admirals and as a Dark Side Adept. He was created by Daniel Wallace and Abel G. Peña for the article "Who's Who: Imperial Grand Admirals" in Star Wars Insider 66 (April 2003), and he is a Star Wars Legends character.
What distinguished him was a talent for using the Force to weld the units under his command into a single co-ordinated instrument. He was taken to the Sith world of Dromund Kaas and trained by the Prophets of the Dark Side before returning to naval service, and he applied battle meditation on a fleet-wide scale at Endor. He died aboard the second Death Star when it was destroyed.
Catalogue entry. This figure has a verified record here rather than a full write-up — the encyclopedia labels its own depth instead of padding it.