Diabolos

Final Fantasy VIII
Diablos is a Summoned monster that appears first in Final Fantasy VIII when characters are given lamp. The lamp has instructions not to rub it... unless you're absolutely sure you want to. Upon rubbing the lamp, the screen will disolve and characters will have to immediately fight Diablos inside of his lamp. If they they beat Diablos, they gain him as a Guardian Force, and if they do not, they had better hoped they saved relatively recently.

Final Fantasy XI
Diablos is also part of the Final Fantasy XI Chains of Promathia storyline. He is a very challenging encounter because he has the ability to Poison and Sleep the entire challenging party even though that is against the rules. Players who have defeated Diablos, as part of the storyline and then go back to face him again can earn the right to use Diablos as a player summonable creature.

Diablos' abilities mostly effect a a party member's magical attack and defense bonus, as well as degrading the enemy's resistances to magic.