freyar
Extradimensional Explorer
Note they also get +2 Str every other head/CR, +1 natural armor every head/CR, +1 fast healing each head/CR, and feats per HD. So we could stick with what I suggested or go with something more uniform, like +1 natural armor each time and +2 Str at 5 and 7 tendrils. I think I like what I put down, since it gives them a little extra (after all, they're not getting extra fast healing).Same guy chiming in again, but I compared to the hydra and they don't gain much more beyond an additional attack at each increased CR.