The following contains spoilers for The Acolyte episode "Destiny ... who reportedly did that was Darth Plagueis, Emperor Palpatine's former Sith master. Palpatine mentions this to Anakin Skywalker ...

Vader has been distracted and weakened by the Emperor’s Sith ... who truly knows that Palpatine is also Darth Sidious, and just how formidable the dark lord’s Sith powers are.