The functionality described is not possible except with Righteous Defense.
Originally Posted by Aberen
You can taunt the "target" of a friendly unit. But you can't gaurentee their target is the one that is attacking them. For example if a healer pulls aggro off you while targeting you to spam heals on you, there is no functional way for you to reference the unit your healer pulled aggro on relative to the healer, you only see them and their target.
The best way to do this is to use a threat catered nameplate system that changes the nameplates based on your threat situation and then you can target and taunt the mobs you lose by the name plate. One such example being Tidy Plates - Threat Plates.
Righteous Defense functions with server sided scripted rules which actually looks at the threat table of the target to determine which targets it applies the taunt to. It is the only taunt in the game that functions like this where it actually can look at the threat table to derive its target. No client side scripting can dynamically do this.
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