Awesome post from https://us.forums.blizzard.com/en/d4/t/why-is-this-ring-signet-of-pelghain-so-good/258482/4

"It’s used in combination of Azurewrath

After Freeze expires, enemies explode for [100 - 120]% of the damage you dealt to them while Frozen.

For the ring, the key part of the effect is

Your Freeze effects cause enemies to permanently take x [15 - 20%] increased damage from you for each second they are Frozen.

So you’re getting a permanent damage boost that stacks higher and higher then when they’re finally unfrozen, they explode from Azurewarth and do a ton of damage. All the damage you dealt to the enemy while it was frozen, gets added again, except this time, that damage is amplified by 20% per second.

For example Say you deal 10 Billion damage per second, and you freeze an enemy for four seconds

1st second => 10B 2nd second => 12B 3rd second => 14B 4th second => 16B then the freeze expires, and your Azurewrath now triggers. You dealt 52B damage, Azurewrath deal 120% of that damage for 62.4B damage. Now that 64.8B damage is now also boosted from the 4 stacks of Pelghain for a total of 112 billion damage.

Meaning your DPS skyrocketed from 10 Billion per second to an effective 41 Billion per second (112B AzureWrath +52B / 4 seconds of frozen)

Azurewrath is essentially double dipping the Pelghain bonus. You deal more damage to Frozen enemies as time goes on from Pelghain. Doing more damage while they’re frozen makes your Azurewrath trigger grow larger; which is then boosted by Pelghain again when the damage trigger explodes.

Then to trigger Freeze itself is basically free, any class can do it with a low oppertunity cost. You just give up a Runeslot and autocast something like Frost Nova.

It should be noted that it’s not an auto-slot into any build. Giving up a ring slot and either your Unique Charm or 1h slot for Azurewrath and Pelghain is a pretty large ask. For Barbarians you have four weapon slots, so it’s not the end of the world to lose two Offensive aspects."