Living.
To want to struggle to your hardest, no matter how it would impact the others around you just to protect your life, just so you could see another day's light.
Isn't that just a natural desire?
Alena was desperate.
Without thinking much anymore, she began the process of Possession.
In truth, if Ilyin didn't have those somewhat conflicting, positive thoughts at this time, Alena would've been hit with a pain enough to take her life right now and then.
However, because Ilyin had been pandering about this matter, her hatred was temporarily relieved and it allowed access for Alena to get into her mind and do the Possession.
The link was then created, and new things became exposed to them.
The true meaning of Possession was actually just gaining the ability to enter your vassal, but Alena and Ilyin's link was much more stronger than that.
