Infinite Dancing Rune Weapon, Blood Death Knight (Gamebreaking) (Patreon)

The blood death knight ability dancing rune weapon

https://www.wowhead.com/spell=49028/dancing-rune-weapon

can be extended indefinitely.  

This makes the death knight virtually invincible and significantly increases dps.

This ability conjures a phantom weapon which mirrors your melee attacks and significantly increases your parry chance. 

To extend the duration indefinitely you will need the blood death knight 2-set, where heart strike increases the duration of dancing rune weapon by 0.5 seconds and additionally gives you a 1% strength increase per cast. 

Normally a death knight's rune regeneration is slower than is needed to extend dancing rune weapon indefinitely. Without runes you cannot use heart strike and without heart strike you cannot extend dancing rune weapon. 

I have found two days to increase rune regeneration over the necessary threshold:

1.  Ashran "Burning Power" Buff

In the Ashran battleground there is a buff in the west of the zone, in the area populated by elite ogres where the Ancient Inferno and Ancient Artifact spawn.  

A small light emanates from one of the braziers in this area. If you move close to it and click on it this gives you the buff "burning power"* . This buff increases your rune regeneration by 50%. 

If you practice on the nearby ogres you will see that heartstrike can be spammed indefinitely taking your dancing rune weapon duration well over its initial duration. 

Note that the dancing rune weapon buff persists through loading screens/instances, with the exception of arena. 

(Note: the "burning power" buff does not survive leaving Ashran. The dancing rune weapon it creates, however, does)

2.  Deadmines "Captain Cookie" Haste Buff

EDIT: This part seems to be fixed. Multiple reports of the buff being removed on exit from the instance.

In the Deadmines dungeon instance the last boss, Captain Cookie, throws food at the player. The "good" food gives the player a haste buff which stacks up to 3000%. For blood death knights haste increases rune regeneration.

You need relatively few stacks of this buff to exceed the necessary threshold to extend dancing rune weapon indefinitely. There is a bigger problem though-the lack of a target. A Turnip Punching Bag from Pandaria

https://www.wowhead.com/item=88375/turnip-punching-bag

can be used for 5 minutes which will allow you stack dancing rune weapon for 3 minutes. 

3. Other Hypothetical Methods

In theory the rune could be extended by simply maximizing player haste. Haste increases the speed of death knight rune regeneration so in theory a haste-optimized build could eliminate the need for special buffs. So far I have not managed to exceed the necessary threshold but there are so many potential ways to do this I have barely scratched the surface of the problem despite working on it for several days. Theorycrafters may wish to investigate this themselves-if the threshold can be broken you could essentially tank anything.

Torghast may also be an option here: however blood death knight anima powers do not seem to have any haste or rune regeneration-based effects (provisional: I could be wrong about that). 

Note that the blood death knight 4-set causes  an automatic heart strike on anything which damages the player with a probability equal to parry chance . With 40% extra parry chance from dancing rune weapon this becomes severely broken. 

* This may be broken for other specializations also. I have heard in particular that this buff  is  very powerful for resto druids. I found it to be strong for arcane mage (it regenerates mana very fast).