Necropotence is a light armor set. While you have a pet active, your Max Magicka is increased by 3150.
This set increases your max magicka as long as you have a pet active. It is a very popular set among Sorcerers and Wardens that use pets. Pets scale better with max magicka, therefore Necropotence will increase the damage of your pets by a lot. The 3150 max magicka cannot be modified, which means that even if you have increased max magicka modifiers active, you will still only receive the flat 3150 max magicka.
The Necropotence set is part of the base game and drops in Rivenspire in the Overland content. You can also find it on Guild Traders. Each set item is bound on equip. You can acquire light armor, weapons and jewelry of this set in the Argonian motif style.