Ring of the Troll Magi
Ring, very rare,
requires attunement, cursed.
One of the first trolls created was able to innately use Chaos Magic. This pleased Orcus so much so that he crafted a ring for those trolls who showed the most proficiency with the art.
When one of these rings is worn by a troll shaman, it converts all fire damage taken to necrotic damage. When the troll is struck with a fire attack, the ring glows a sickly purple, and where a burn wound should appear there is instead a patch of festering rot.
If any creature other than a troll wears one of these rings. it converts not only fire damage but ALL damage to necrotic. In addition, once a creature has taken a cumulative total of converted necrotic damage equal to what their hit point total was when they very first put on the ring they transform into a troll who is immediately an NPC monster under the DM's control.
If a character attempts to identify the item before anyone puts it on, they learn about everything except the ring's ability to transform non-trolls into trolls. Once put on, a character has no desire to ever remove the ring, as it is cursed. Remove Curse can either remove the ring from a character's finger, or bring a transformed troll back to their original form.
I decided to not make it require attunement so that the curse is immediate on anyone who puts it on.