The last boss in Pyramid of the Ancients.
You need all four gems (Corruption, Piety, Fire and Frost) to finish this fight, unless you are overpowered for the dungeon. (He'll do about 2000 dmg per hit, for his attuned element). If you don't have them, check Pyramid of the Ancients for more info on how to get them.
You'll open the fight against the Oracle. Just DPS him down and Lord Atum-Keket will eventually appear. IMMEDIATELY use your unholy gem, as that will be his first "deadly element".
If you watch Atum-Keket's buffs carefully, you'll see he is super-attuned to a single element and switches each time he loses 25% health. Order: Corruption (Unholy), Piety (Holy), Fire and Frost. Your gems will buff you to 100% invulnerability from that element (meaning the spell will actually HEAL you if you already have invulnerability of that variety). Spells cast from any other element will do about 100 damage (depending on level). If your video settings are high enough, you can see his element changes by the color on the aura on him.
He teleports and spawns some minions. The only thing you really need to worry about, is having the right protection buff up at all costs.
Atum-Keket shifts his attuned element roughly in this order:
- Health at: 75-100% unholy
- Health at: 50-75% holy
- Health at: 25-50% fire
- Health at: 0-25% frost