From personal experience (as a RR81 BiS RP player);
•when you try to cast it someone will die because you were not healing for an extended period or because you were moving into range to use it
•if you are trying to rez 3+ people in your group you have probably already lost and are either running or about to die yourself (assuming your group died next to each other in the first place)
•the places it would be extremely useful, like chokepoints, it is irrelevant because most if not all of the dead will not be in your group
•moving into range to use it will likely move you into AoE or worse make you a focused target, which will at a minimum will increase the already exceptional cast time due to setbacks
But these shortcomings are necessary for AoE rezzes as they are extremely powerful and hard to balance, imagine trying to take a fort lord room if each Runepriest could rez dozens at a time. Instead of trying to fiddle with the unusability of Grimnir's Fury, which in practice leaves Runepriests without a 13 point ability to spec for in what is their most important mastery line, why not just replace it with a functionally usable ability?
Which leads us to the question; If you replace Grimnir's Fury with something usable, what do you replace it with?
-Let me start by saying that healing Runepriests are strong and in a good position, they synergize well with Warrior Priests. Because of this I would not suggest replacing Grimnir's Fury with an ability for healing Runepriests but rather an ability for dps Runepriests. It is a niche spec but dps Runepriests are still desired for warband play due to the unique armor debuff they bring. It is not a 'selfish' spec either as dps builds using the AoE mastery are based around group play instead of soloing, with less of an emphasis on lifetap healing.
-Secondly as an ability in Runepriest's AoE mastery the ability should be AoE in nature. We would need to be careful with what type of AoE ability to give because Order already has strong ranged AoE dps from Bright Wizards, Engineers and Shadow Warriors. Any ranged AoE ability would need to have limiters either via cool down or cast time.
-Thirdly if this dps ability has attached utility (debuffs, drains or CC) it would need to be made in such a way that healing Runepriests would not be inclined to spec for it. This could be done through various means such as having the utility be minor, having the ability be put the Runepriest in danger to use or by gating the effectiveness of the ability behind tactics not used when set up as a healing Runepriest. This also effectively rules out having an Elemental resist debuff component, which would belong to the tactic Concussive Runes anyway in order to mirror Runepriest's counterpart class the Zealot.
-And lastly let me add that due to the nature of Runepriest lifetap healing I would be hesitant to suggest a spammable ability that does direct damage.
With that all said I would propose the replacement ability to be a melee AoE channel with the same mechanic and damage of Wind of Insanity, the 13 point ability in the same mastery line of Runepriest's counterpart class the Zealot, except with a snare effect instead of a knock back effect.
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Grimnir's Fury
Damaging
Path of Grimnir
Level 29
20 Action Points / Sec
Self
Instant cast
26s cooldown
You securely root yourself in place and summon your ancestor's fury which inflicts 172 Elemental damage every second and snares by 40% all enemies within 30 feet of you for up to 6 seconds.
This spell may be toggled off to regain your movement.
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The damage would be elemental like all Runepriest attacks and the snare effect would fade either when the channel ends or the enemy moves outside the area of effect. This would also allow reuse of the current spell effect of Grimnir's Fury as the runic circle would be the indicator of the abilities activity.
