Alastor is an undead monster that is armed with a staff. You encounter him in the Athenian Catacombs - Level 5.
Alastor has five abilities, some similar to the Spirit Mastery's skills
- Staff Attack - Normal ranged staff attack. Staff elemental type is random. Lightning Damage, Cold Damage or Fire Damage
- Deathchill Aura - Just like the Spirit Mastery, Alastor has his own version of Deathchill Aura. Slows attack speed. Cold Damage & Vitality Damage
- Spectral Bolt - Similar to the Liche King summon, Alastor also fires a ranged blast that reduces health by a percentage. Cold Damage & Vitality Damage
- Summon Skeletons - Summons skeletons that deal moderate damage. Cold Damage & Physical Damage
- Life/Mana Leech - Leeches away at life & mana
Alastor is immune to poison and bleeding damage and appears to be either highly resistant or outright immune to piercing damage as well. He is very difficult to tank, as he and his minions have a very high damage output that can kill you if you stay in one spot too long. One thing to remember is to never kill all of his minions. Here's why: He starts out by summoning three skeletons. However, if you kill any of them, he will summon three MORE, which means you are now facing five skeleton minions. If you can get your cold resistance to 40-50%, this fight will be much easier. You don't have to worry too much about his life leech ability. Get vitality resistance if you want, but cold resistance is much more valuable.
Pets are almost useless against him (especially on Normal difficulty), except for the Wisp and the Nightmare, which have a chance to dodge his attacks.
The safe way: You can actually fight him from the door using a bow or spells (hold Shift to lock movement). This way the door won't close and you can back up any time to recover. He usually won't react to your attacks but instead keep on moving around randomly. It can be a bit tedious to fight him this way, as he constantly moves out of range (except when attacking with "infinite range" attacks such as the Lightning Bolt).