He is a vampire king, who has also killed a number of past Slayers and was apparently a vampire to be feared. Lothos has a eccentric right-hand vampire named Amilyn, that has aided him throughout the centuries, killing any activated Slayer when they get the chance, but the TV series implies that at least a handful may have elluded him.
At the time of the movie, Lothos has a gang of vampires that began terroiseing Las Angeles, being responsable for the deaths and dissapearances of many teenagers, shortly after Buffy Summers is revealed to be the new Slayer. One of Lothos' latest minions is the newly turned dropout named Benny,who is friends with Oliver Pike, who attempts to get Pike to join him.
While Buffy is trianing with her original Watcher, Merick, Lothos sends Amilyn and his minions to confront her. At this time an upcoming dance is being prepared and Buffy has killed all of Lothos' minions she encountered before they can tell thier master where she is. At one point Buffy, Merrick and Pike encoutner a large number of vampires in a forest and engage in a fight with them, in which Amilyn loses his right arm, and Lothos doesn't seem to react to Amilyn's injury as he still has his sights on Buffy.
At a basketball game, a classmate of Buffy's, who was turned into a vampire, attempts to warn Lothos of her presence, after a lenghthy chase, Buffy meets up with Pike and slay the vampire along with more of Lothos' minions. In the confrontation, Merrick attempts to stake Lothos but the vampire turns the tables and impales Merrick with the same stake, before leaving saying Buffy "Is not ready".
Buffy starts to lose faith in herself as a Slayer, much to Pike's dissapointment. Over the next week as the dance draw's nearer, Lothos hasn't made any appearances. On the night of the dance, when Pike and Buffy talk to eachother and start to dance with each other, Lothos leads his remaining minions striat to the dance, where they start to attack. Pike and Buffy fight off the vampires that are attacking the prom and Buffy decides to confrot Lothos once and for all. Outside the gym, Lothos draws Buffy into her power after Amilyn is mortally wounded. While Lothos is close to killing her, the lights go out (Caused when Pike kills Benny), catching them both by suprise. Buffy takes her hairspray and makes a makeshift flamethrower and burns Lothos' face. Buffy heads backt to the gym with Lothos' army all dead but Lothos emerges again wielding a sword and attacks Buffy again. After Pike fails to help cause Lothos any harm, it becomes apparent that only Buffy can kill him. When Lothos is caught off guard, she stakes Lothos and kills him.