"Zender the Greater Daemon" appears on Adventure Card #13 in the City of Greyhawk boxed set. Because this was a very early 2nd edition scenario (before daemons had been updated to 2e stats, and even before they had been temporarily banished from the game, but after the old Fiend Folio and Monster Manual II had gone out of print) his exact type isn't mentioned, but from his stats I assume he's an ultroloth (he's got one more hit die and 20% better magic resistance than the 2e ultroloth, though, and his spell-like abilities are somewhat different, though overlapping). His true physical appearance isn't described at all.
His main thing is that he's captured the archmage Bigby and is, at the time of the scenario (581 CY or thereabouts), posing as him. I guess it's assumed that the PCs eventually unmask him, but it'd be amusing if they didn't, and the real Bigby is still imprisoned beneath his tower somewhere. If he's still impersonating Bigby in 596 CY he'd probably have had to have cloned himself, though, or convincingly pretended he had.
He's served by a lesser guardian daemon (called simply a Lesser Daemon, but his stats are identical to the 2e lesser guardian yugoloth).