Aesgareth, a Cambion who likes to gamble. Portrait from the PPE Mod
Aesgareth is a demon (chaotic evil 
Baldur's Gate II:
Throne of Bhaal (2001)
This icon indicates content from the original Baldur's Gate II: Throne of Bhaal campaign. / chaotic neutral 
Baldur's Gate II: Enhanced Edition – Throne of Bhaal (2013)
This icon indicates content from the Throne of Bhaal campaign of the Baldur's Gate II: Enhanced Edition.)
who at some point left his own Plane and ended up getting trapped inside Watcher's Keep.
On the last area of the third level of Watcher's Keep, you will find Aesgareth. He will offer to play a game with you involving the Deck of Many Things. You can gamble for a few rare items (a scroll of Wish or the Spectral Brand), as well as a gemstone required to leave the level.
If you win the gemstone from him, he will attack your party in order to claim the Scepter of Radiance and escape. However, if you end up losing the scepter to him, and allow him to have it, he will keep to his word and give it back to you (as well as the gem you require) once he has freed himself.
The first and second rounds of the game are a toss-up, either you or Aesgareth can win. However, if you play until the 3rd round, he will insist on playing only for the scepter, and he is guaranteed to win.
Note that the only way to claim the Deck of Many Things is to fight Aesgareth and win, where the deck will be left behind in his possessions. If you allow him to leave, the deck will be lost to you forever.