New season, new toy!
[Formula: Enchant Weapon - Deathfrost]
Drops from Frost Lord Ahune, BoP, costs 2 Primal Water and 2 Primal Shadow.
"Permanently enchant a weapon so your damaging spells and melee weapon hits occasionally inflict an additional 150 Frost damage and reduce the target's melee, ranged, and casting speed by 15% for 8 sec. Requires a level 60 or higher item."
A paladin tested it with and without SoCr (40% melee haste Seal) with an 1.6s Mace (S2 Gavel) and got an 8% proc rate both times. A hunter did a few tests and got ~8% too.
That was what I picked up in guild chat at least. Unconfirmed data is better than no data.
The proc can crit (with your spell crit rate probably), crits for 150%. The
[Chaotic Skyfire Diamond] seems to affect it, the "200% frost crit" talentt does not. Zone buffs, Fericious Inspiration, etc. affect it.
So, proc rate, PPM/percentage and cooldown would require some log to check properly.
Could use a test whether it procs in feral forms or not as well.
I expect a lot of ogre spirits, demonic servants and megalomaniac doctors to be hurt in the process.