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"Why build a Tower to place the beast below ground? At times I believe the Horadrim lacked common sense."
- Hratli

The Guardian Tower was built in Kehjistan to contain Mephisto. The Lord of Hatred himself was actually imprisoned three levels underground within the Durance of Hate, while the tower is said to have risen three levels high.

The tower was formerly known as the Temple of Light, but after being tainted by Mephisto, it became known as the Blackened Temple.

The tower marks the spot where Mephisto was buried. The Horadrim built it with the intention to keep Mephisto there until the end of time, allowing the Skatsim to guard him. The Horadrim didn't plan on Diablo escaping or the Zakarum taking over.