Alastor is considered for Lucifer's right hand man. In the original Painkiller Alastor is appearing as a chapter boss, at the end of the fourth chapter. After killing him, in cinematic cutscene, Daniel is staying to his dead body with Eve, when Lucifer himself is appearing. He kidnapping Eve in Hell. Samael is arriving, saying that he has done his job and now he can enter the Heaven, but Daniel refuses. He want's to kill Lucifer, so he goes to hell. After killing Luciffer in cinematic cutscene, Daniel being surrounded by hundreds of demonst, leaded by Alastor himself. Daniel is stunned from his appearing. Alastor is starting to explain a lot of things, the Luciffer was inloved in Eve and this became his weakness. But as a master of Hell he was the only one, who can give an orders to the Hell armies to attack, and he was slowed his plans, because of Eve.
After this Alastor is giving order to his demons to attack Daniel and this is the ending of the original Painkiller.
There is another, second ending, accesable only in Trauma difficulty, in which Alastor serves as final boss, and when defeat him Daniel is entering the Heaven to his wife.
Battle out of Hell continues the normal ending. The game begins where original Painkiller ends - Daniel is alone in the middle of Hell, when Alastor says to his minnions to attack him. With last powers Eve is opening a gate for him to escape, so Daniel takes her, and uses the teleport. But he can't leave Alastor alive, so he decide to get back to hell and kill him once and for all. Eve is showing him alternate way to reach the Hell and telling him when he kill Alastor to stay away from his dead body, because the first who touched it will became the new ruller of Hell.
In the ending, after hard battle, Daniel is defeating Alastor and while staying, heavyly wounded over his dead body Eve is appearing, revealing her true plans. Using all the time Daniel, to clear the Hell, he touches Alastor's dead body and take his powers, becaming the new Hell Master. She is offering Daniel to join her, but he refuses. Eve left him alive, but Daniel rises his shotgun and blasts Eve from close range in back. This is how Battle out of Hell ends.
In the final hall of the Tower there are a lot of statues arround the arena. When they start to heal the wounded Alastor they became vulernable and this is the only moment when they can be destroyed. Destroy all statues, so Alastor can't heal, and after this kill him.
There are sparkling circles spread on the arena. Step in one circle, harm yourself (using single flame shot from the flamethrower) and will appear a stone soldier, who will hit Alastor with hammer, making him vulnerable for short moment. Use this moment to kill him. If you not succeed with the first try, repeat all again.
There are no tactics in defeating him. Just shoot it until he/she dies.