Thranduil, by and large, is unflappable. He has dealt with many dangers for a very long time, and made merry in their spite. Despite this - and possibly for the first time Legolas will easily be able to recall, or be able to remember at all - and certainly for the first time in a very long time, Thranduil looks alarmed, almost scared.

"What do you mean it keeps your soul intact?" His voice is sharper than it normally is when there's nothing actively trying to kill him or those in his charge, as are his eyes.

He really doesn't like the unknown.

This, combined with all the other things - this place, the differences in Legolas's appearance, the thrice-cursed collar, is making him doubt himself. He reaches out to touch it.