He quickly moved his hand up and removed the shard from Cas's neck with brute force
Cas: AHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHH!
He passes out
"..."
He floated out of cas's body and grabbed the shard, inserting it in his own neck, wincing a little.
Cas gon dieeeeeeeeeee
"Hm..now replace with weaker shard.."
He took a plain white shard and pushed it into Cas's neck
Cas wakes up with a start at the pain
Cas: AHHHH!
"Yay."
Cas: W-what...what did you do to me...?
He's shaking, clinging to Elden
"Nothing."
Cas shakes his head
Cas: No...no...I...
Cas goes limp
Cas: Nnng-
Cas: ...
He shows Elden his neck. The red shard got replaced with an extremely weak plain white one
Cas's face is covered in sweat, dark circles under his eyes
Cas: Nnn...E-Elden...I-I don't...I don't feel good...