After all, there was still a massive amount of Evil God power inside Geralt. To the Chaos Gods, he was a very high-capacity battery. Well, the reason they considered him a battery rather than thinking their power had gone down the drain was that before draining the Chaos Gods' power, a specific backdoor had been left in that technique. It gave the Chaos Gods the illusion that their money—well, their power—was still under their control, allowing them to safely invest more power and continue corrupting Geralt until their own power completely took the advantage and turned him into a thorough vessel.
And incidentally, act as a battery.
That power still looked as though it belonged to them, but that was only on the surface. In reality, their power had already been locked down and could not be withdrawn at all.
Chapter 260