Attack Power Buff - Works In Mythic Dungeons (Alliance) (Patreon)

In Howling Ford, Valgarde, in Northrend, there is a quest chain which begins with Vice Admiral Keller's "Hell Has Frozen Over". Three quests in you should then receive "Prisoners Of Wyrmskull" https://www.wowhead.com/quest=11255/prisoners-of-wyrmskull

This quest requires you to free prisoners from cages using keys found on the bodies of surrounding hostile npc's. 

There are four types of prisoner, the ones you are interested in freeing are paladins. The paladins cast a spell on you when they enter combat (they fight with you for a short while) "Greater Blessing Of Might". This is an old spell that used to be in the game in early wow but was removed. 

The tooltip states that it gives your character 10% attack power and lasts 30 minutes. Exactly how this affects your characters stats in actuality given the complicated interactions between a Wrath buff and Shadowlands scaling is any one's guess, but it increased my death knight's damaging abilities by around 20%, which is significant. Patreon Thomas below reports a 10% increase in physical damage from combat logs, which is still significant but obviously there is some discrepancy.

I took the buff into battlegrounds, dungeons and even mythic dungeons without it falling off. 

The priest prisoners on the same quest give you a one hour buff called "Prayer Of Fortitude" again, a 10% stamina buff from early wow. It increases your health but does not survive a port. Both buffs are worth having if you level in Northrend.