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Molten Colossus is a mob found in Ulduar.

Normal mode (10-man) abilities

Earthquake Inflicts 6013 to 6987 damage to nearby enemies and knocks them down, silencing them for 4 sec.

There are reports that this does not affect players if they are out of line of sight.

Pyroblast Inflicts 8325 to 9675 Fire damage to an enemy and scorches the target for an additional 3000 damage every 3 sec. for 12 sec.

Tank and healer placement

Split the raid into two groups. 1 consisting of 1 tank, 1-2 healers (Depending on the setup) and all of the DPS. The second group consisting of 1 tank and 1-2 healers (Depending on the setup).
Tank the two molten colossi at the two corners before the stairs.
Have the healers stay as far away from the corner as possible, leaving them out of line of sight of the molten colossus, thus leaving them unaffected by Earthquake
Make sure that tank and DPS is always in line of sight of the healers, but not the molten colossus.

Hotfix (2009-05-20): "The Molten Colossus will no longer cast Unquenchable Flames. ^[1]"
Hotfix (2009-04-21): "Players should no longer be able to quench the Unquenchable Flames debuff from a Molten Colossus by using spells like Cloak of Shadows or Divine Protection ^[2]"
Hotfix (2009-04-17): "The normal mode Molten Colossus have had the cooldown of Pyroblast increased. ^[2]"

^ Ancilorn 2009-05-20. 20/05 Ulduar Tuning Tweaks. Retrieved on 2009-05-22.
^ ^a ^b Bornakk 2009-04-17. Recent In-Game Fixes - 4/16/09. Retrieved on 2009-05-24.