It can be obtained by participating in the second world event, at a cost of 32,000 renown, requiring a runite event token. After the event is over, it can be obtained as a very rare drop from General Graardor and Kree'arra. When received as a drop from the generals, it is automatically learned; it does not drop as an item. When received as a drop, the message You have unlocked the Devotion ability! appears in the chatbox.
Devotion can be very useful in the following cases:
Devotion will block all the attacks from all of the Ascension bosses (including the Lines of Ascension from Legio Quintus).
Devotion will block all the damage from Nex's magic or melee attacks, however it will not protect against the shadow traps, the blood sacrifice attack or the ice prison attack (It will also block the attacks from the minions).
Devotion will block 75% of the attacks in PvP combat.
Devotion will block Dharok's "Give me everything!" attack and also blocks his increasingly damaging attacks as his health decreases.
Originally when this ability was released the cooldown time was 60 seconds. As of the patch notes of February 24th, 2014, this was changed to 90 seconds.