When a Great Old One Warlock uses the Create Thrall feature (PHB, p. 110) in Adventurers League play, does the Thrall work with you? Does it persist through sessions? Is there any guidance on how this works?
The Detect Thoughts document hosted on dndadventurersleague.com is a compilation of rulings made by AL admins and such, and the latest version (1.4) says:
Do spell effects carry over between AL sessions?
The AL guidelines do not currently hold an official position, so this will be a decision between players and their DMs.
Of course, Create Thrall is a class feature and not a spell, but I can't imagine the situation is any different in the absence of a ruling that says otherwise (like the guidance on Wish, for example). Having the effect of create thrall persist over multiple sessions only really makes any sense in the context of a hardcover where the same NPC/creature you've made into a thrall is going to be present session after session. In that case, I suspect most reasonable DMs will let the effect persist as long as you remain in that hardcover.
On the other hand, playing a module and trying to make use of a thrall you created outside of that module would be very strange, and I suspect most DMs would not allow it (even if it was a module in the same setting as the original use of Create Thrall).