Bardolph Littlejohn QC was Sabrina's enemy in 20th-century England (one of several, it must be admitted). He was a barrister representing capitalist corporations which encouraged environmental destruction, while she was a militant environmentalist.
Littlejohn is now a priest worshipping the Titans. Exactly how this came to pass is not particularly important. His outlook on life has not changed much. He still tells lies, and he still believes in greed and environmental destruction. All that has changed is that he represents evil Titans rather than evil corporations.
[AD&D-Specific:] Bardolph Littlejohn, human male, Str 13 Dex 13 Con 11 Int 18 Wis 15 Cha 17 Align NE Hp 61. Priest, level 17; Enchanter, level 8.
Unless you have a specific mythos prepared for evil Titans, use the "Priest of Trickery" from the Priests' Handbook, with no access to 7th-level clerical spells as Titans are not major gods. This last point does not affect Littlejohn, who lacks the wisdom even for 6th-level clerical spells.
Littlejohn is perfectly at home in Atlantis, where there is easily enough everyday lying, greed and environmental destruction to suit his tastes. An additional bonus is that living in Atlantis gives him the opportunity to study enchantment magic. He is a member of the Learned Society of Conjuration, Binding and Enchantment and from time to time improves his skills by taking short courses at the Institute of Enchantment.
However, the Titans who give Littlejohn his priestly powers do not do so in order that he can take life easy in Atlantis. They want him to hunt down and kill Graeco-Roman priests, starting with Sabrina. In pursuit of this tiresome necessity, Littlejohn may turn up anywhere.
Note for DMs:
Bardolph Littlejohn is quite a good adventure hook in his own right. For example, he might charm a group of peasants into doing deeds of environmental destruction. It is then up to the player characters to stop the peasants, preferably without killing them. (Oh, all right, zap the peasants if you must. Peasants are easily replaced.)
As Littlejohn, like other NPCs, continues to rise in level, it might also be a good idea to do something about him now, before he becomes really dangerous.
However, Littlejohn's main function in the campaign is as a potential nemesis for Sabrina.