Signet of Pelghain is a Unique Ring 1 in Diablo IV Season 15 for All classes. It drops from The Beast in the Ice.
Unique power
Your Freeze effects cause enemies to permanently take [15 - 20]%[x] increased damage from you for each second they are Frozen.
Affixes
Primary Core Stat
All Damage Multiplier
Where it drops
The Beast in the Ice (Initiate Lair Boss, 1x Lair Key) - dedicated multi-class Unique
Can also be crafted into a Unique Charm (Horadric Cube > Craft Unique Charm) per 3.2.1 notes; any Unique Charm has a small chance to drop at Mythic quality.
General Unique Pool - every Lair Boss (Initiate, Greater, Exalted, Belial) can drop it in addition to their dedicated table (one guide's Boss Loot Table Cheat Sheet)