Shadow Word: Insanity Mechanics

Archangel is no longer a talent option (probably because it was garbage for Holy). It has been replaced with Power Word: Solace, which deals a pitiful amount of Holy damage (or Shadow damage in Shadow spec) and restores 2% max mana. If you shift into Shadowform, however, it morphs into Shadow Word: Insanity. Its tooltip lies; see below.

Straight from Ghostcrawler (Source) :

"The damage scales based on the percentage of ticks that have already gone off from the spell (specifically, for each DoT, the damage is increased by (# ticks elapsed / (total ticks – 1)) * 100%, so for a base SW:P which would tick 6 times, SW:I gains 20% damage per tick elapsed), the tooltip should reflect this in a future build. The intention is that you can use Shadow Word: Insanity in solo combat when a mob is getting low to help push them into Shadow Word: Death range, or before your DoTs fall off on a raid boss to eek some more damage out of them. The damage increase based on duration is there to avoid a situation where the right way to play is to apply your DoTs and then blow them up, which we don’t consider fun gameplay."

It's obviously IMBA for PvP, but its PvE uses are less obvious. For narrow bust windows, if any, it may be useful. Depending on its base damage and SP-coefficient, it may also be rotational if/when your DoTs all run low at the same time. If our mana income is too low, then the talent also gives us the option to leave Shadowform to spam Mind Siphon. Frankly, I would be surprised if the talent comes anywhere close to viable when compared to the other options on that tier, but I haven't run the math yet.