As promised in the post about the Krystal Easter Egg in Serious Sam 3 I contacted Anton Antonov Marinov, the 3D Environment Artist artist for Ubisoft, whose developer sign in the game featured Krystal. I had heard he put another, similar Krystal Easter Egg in another game, and he confirmed to me that it was in the The Talos Principle, a first-person puzzle game.
In The Talos Principle, there's a secret "developer island" where among other things, you will find sign posts from various people who worked on the game. Anton likes to mention Krystal in his, and this is no exception:
Anton "Mischievous" Antonov Marinov
He got to live forever after all.
Took the frogs and the rocks
and the trees and Krystal and
all of the other persons with
The icon of Krystal featured on the sign is actually a mirrored version of '...and you need Krystal's help' by Cammiluna, one of the older Krystal fanarts in the world (April 2, 2005).
Check out the video below to see where the sign is located in-game. Video by Marble Entertainment.
(PS - While researching this post, I found out that Mr. Marinov has commented here on the Krystal Archive before. But only once, in 2011, probably while he was working on Serious Sam 3.)