抜け目ない潜り手、ニアディヴ

If Neerdiv leaves the battlefield while its first ability is still on the stack, use its power as it last existed on the battlefield to determine how many cards the target player mills.

Neerdiv's last ability resolves before the spell or ability that caused it to trigger. It resolves even if that spell or ability is countered or otherwise leaves the stack without resolving.