He was referring to this basilisk https://en.wikipedia.org/wiki/Roko%27s_basilisk

In short, this is the believe that a god-like AI could punish them retroactively, for not having done all that was in their power to create this AI.

(A bit similar to some religious believe that a god could punish you after your death if you did not spend your live "pleasing" said god during your life)