what is a skill horcrux

[tyber nexus]

One of my SPs is almost maxed out and I noticed there was a present next to her in the list. I clicked it and now i have a skill horcrux ... what is that?

[Imperial Sergius]

Hello.
In a hero info menu you have the heirloom tab.
In this menu a hero until the rank SS have only a heirloom socket, but a SP rank hero have an additional one for a horcrux.
What I do not understand it is why horcrux is a present for SS heroes, but they can not equip it. Is it a mistake of developers? Does exist a way to evolve a SS hero to SP?