Valkorion does this to them:
Originally posted by S_W_LeGenD
When?
"We must deal with my errant son and daughter before they ruin everything."
"I knew you wanted something. You can't stop them alone."
"They are deadly - and worse, lack discipline. do not underestimate their threat. It will take both of us to undo the damage they can inflict."
―Valkorion and the Outlander
---
"My son is too strong. You need my power. Only together can we strike him down."
―Valkorion
Originally posted by Azronger
"We must deal with my errant son and daughter before they ruin everything.""I knew you wanted something. You can't stop them alone."
"They are deadly - and worse, lack discipline. do not underestimate their threat. It will take both of us to undo the damage they can inflict."
―Valkorion and the Outlander
---
"My son is too strong. You need my power. Only together can we strike him down."
―Valkorion
Pretty sure it was explicitly Valkorion's power, given that Arcann notes after the exchange that his father's power has changed/gotten weaker, and that even if you choose a non force sensitive, the result is still the same. Plus Valkorion's the one who essentially gave the Outlander the power necessary to defeat Arcann in Chapter 16.