Nuked from Orbit (25 player)

It's the only way to be sure...

Kill the Flame Leviathan on the third tier of hard mode on heroic difficulty.

There are 4 defence towers in the gauntlet leading to the Flame Leviathan which can be activated by talking to the Lore Keeper of Norgannon before talking to Brann to start the gauntlet. Activating a tower and leaving it intact will activate a unique Orbital Defense System on the Leviathan. Each Defense System grants the Leviathan an extra ability, increases his health by 50% (multiplicative!), gives him a unique buff, converts all dropped Emblems of Valor to Emblems of Conquest and increases the number of Emblems. Killing the Leviathan with 3 Defence Systems active is the third tier of hard mode and allows the raid to get the Heroic: Nuked from Orbit achievement and 3 Emblems of Conquest.

The Leviathan gains 238% HP and three of the following abilities when 3 Defense Systems are active:

• Tower of Storms: Thorim's Hammer - Physical damage. Extra damage at point of impact. Also increases physical damage by 25%
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• Tower of Flames: Mimiron's Inferno - Fire damage and a AoE damage zone. Also increases fire damage by 50%
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• Tower of Frost: Hodir's Fury - Frost damage and entombs vehicles near the impact point in ice. Entombed vehicles are stunned for 1 minute, taking damage over time. The tomb can be destroyed with ignited Tar. Also slows players for 20%
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• Tower of Nature: Freya's Ward - Nature damage and summons Writhing Lasher and Ward of Life adds. Also descreases fire damage taken by Leviathan by 10%

HP and loot results of leaving up fewer towers with Hard Mode active:
0 Towers | 1 Towers | 2 Towers | 3 Towers | 4 Towers
HP [mln] 70 | 105 | 158 | 236 | 354
Emblems 1 | 2 | 2 | 3 | 3
Loot normal | normal | normal | normal | Normal + hard

More towers also means more runed orbs and greater chance of val'anyr fragments.