If I'm reading the final scenes of the episode correctly, I think that the tree (which sprouted up from Number 7's corpse) has become more of a direct antenna/interface with Sol in the core than we've ever seen on this side of the planet before, and with the tree's (re-)growth Sol has become powerful enough to drop the EMF field and make it snow over here now, like the opposite end of the planet. Sol, communicating now more directly via the antenna-tree, reached out to Lucius and egged him on to bring Marcus (and the punisher helmet) here to the tree, crucify him, and "kill" him and... I think Marcus might
actually be dead. I think the Sol signal has supplanted itself, or at least part of itself, into Marcus' body.
On top of the religious allusions, I love the imagery of the upside down crucified floating Marcus as an anthesis/affront to Mother's more "proper" Necromancer form.
Also worth remembering is this Mithraic painting from the Caleb/Mary flashback in Season One Episode Two: