Blizzard have temporarily disabled the Unique power of 
The Umbracrux. The item was generating a whole lot of experience when killing certain bosses, and its power will remain disabled until Blizzard can fix the unintended interaction. This may be related to the recent reports of players getting a whole lot of levels in certain Strongholds we covered here.
Hello all –
The team was made aware of an issue where players were able to obtain a significant amount of experience while killing certain bosses with ‘The Umbracrux’ unique equipped.
We will be disabling the unique power for this weapon while we work on a fix for this. This means players will no longer be able to spawn a shade totem with this weapon. Players can still find this weapon and can technically still equip it, but they will not be able to utilize the unique power while we have this disabled.
Once we have a finalized fix, we will re-enable this and communicate this out to players.
Thanks for all the reports on this issue. We hope to have a final fix for this soon.