He is Bane's foster-father; when Ishtier and Ganth Kendar mistress came down to Tai-tastigon from the Keep, he married her, not knowing that she was already pregnant. Bane was raised as his son.
Theocandi took Bane as his apprentice six years prior to God Stalk to get Abbotir's support in that election, and in the upcoming election in God Stalk, Abbotir is considered a dependable supporter of Theocandi again for the same reason.
↑God Stalk, Book II: Crown of Nights, "Chapter 9: A Matter of Honor" — " 'The other, once mistress to the old lord himself, was taken to wife by a high official of the Thieves' Guild. What no one knew then, or has guessed to this day, is that she crossed his threshold already quick with child.' "
↑God Stalk, Book II: Crown of Nights, "Chapter 5: Winter Days" — " '[Bane] was forced on Grandfather by his father, Abbotir of the Gold Court, just before the Guild Council meeting six years ago in return for political support.' "
↑God Stalk, Book II: Crown of Nights, "Chapter 8: Voices of the Past" — " 'Theocandi can depend on Abbotir of the Gold Court because of Bane,' "
↑God Stalk, Book II: Crown of Nights, "Chapter 11: The Storm Breaks" — "Masters Gold and Shining were Theocandi's; there was no helping that."