Because of this combination of skills / perks
[u]Arcane spirt +divine order[/u] : this combination gives you 80-90% of the bar filled in both armor and recovery( reduses agility to about 15-20%
[u]energy drain[/u]: normally only reduses your grenade cool down but is augmented by other abilities.
[u]Embrace the void[/u]: where it starts to get fun. Damaging enemy's with your grenade / nova bomb triggers energy drain
[u]life steal[/u] killing a enemy while energy drain is active restores a " significant portion " if your health.
[u]the hunger [/u] extends the duration of energy drain
With these abilities you should be able to tank very well
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Best tanks my ass. What you described is a life tank and they are cooldown bound. Titans tank well and then tank better when their abilities are up. Your description makes Warlocks tank well only when they can use lifesteal and energy drain (which isn't all the time)