|Title||The Mad God|
Tarjan is a narcissistic and powerful magic user who is worshiped by many. He is better known as the "Mad God." He is apparently powerful enough to turn a whole city (namely, Skara Brae) into ruins.
From a magic mouth within the Sewers:
Know this, that a man called Tarjan, thought by many to be insane, had through wizardly powers proclaimed himself a god in Skara Brae a hundred years ago...
Tales of the Unknown
In Tales of the Unknown, Tarjan's presence is limited to being a statue atop Harkyn's Castle that comes to life and attacks the party before allowing them to proceed further. During the battle, he becomes a rather dangerous opponent, casting Death Strike and Stone Touch quite regularly.
The Thief of Fate
In The Bard's Tale III: The Thief of Fate, Tarjan reappears as a malevolent deity bent on destroying everything and everyone in the universe. It is up to the party to stop him before time itself is consumed.
Eventually, in Tarjan's home dimension of Malefia, the party has to face the Mad God and defeat him.
Jack Vance's Dying Earth series featured a wizard named Turjan of Miir. The Dying Earth series was a significant influence on D&D, and by extension, The Bard's Tale.
American Computer Scientist Robert Tarjan was the inventor of many highly technical algorithms. The programmers of The Bard's Tale were very likely familiar with his work.