If damage reduces the player to 0 hit points exactly, it must make a Constitution saving throw with a DC of 5 + the damage taken, unless the damage is radiant or from a critical hit. On a success, the player drops to 1 hit point instead.

increases your con + 2

you have 3 charges you can use the form at a cost of one charge,
you can gain by charge by getting a short or long rest.

you crack your bones and then purple glowing energy flows around the cracked area ,
when you attack using this form the energy flows to your attack point strengtening it.
this form last for 3 hours, you can disable or enable it using a bonus action.
once you finish the energy flows to your body and crack your bones into place again.

you get + 1d6 necromanci damage when strengthening your attacks,
at levels 3, 5, 7, 11, 15, 19, 20 you add another 1d6.
add 2 to your AC.
and advantage on strength saving throws or checks