The Flesh-soul platebody is a quest reward from the My Dark Brother quest. it requires level 80 Constitution and Prayer to wield. It has two special abilities, but only one of them can be used at a time. The first one is called "Flesh Leech", the other one is called "Soul Drain". To switch between these two, you need to use at the altar in the Slayerdungeon of Prifddinas while being in the dark realm. Depending which effect is active, it has either named Soul- or Flesh platebody. If you toggle the effect while there are still charges in it, all the charges will be lost. To see how many charges are left, right click on it an select "check charges".