Erik Walpow has rejoined Valve after a year-long post at Double Fine Games. He previously worked on industry classics including Half-Life 2 Episode 1 and 2, Portal 1 and 2 and Left 4 Dead. He departed Valve to join Double Fine Games where he worked on the Psychonauts sequel. Erik also worked on writing the original Psychonauts game in 2005.
His name was noticed by Reddit user Trenchman in the credits of the newly released Artifact. Trenchman emailed Gabe Newell to confirm Erik's position who confirmed the news. Trenchman is certain that Erik has not done any work on Artifact. Erik previously worked at Valve for over a decade following his work on the original Psychonauts. He left Valve towards the start of 2017 only to now have returned back to the company.
It is possible that Erik only left Valve to work on the Psychonauts sequel, given he worked on the original and may have a certain attachment to it. However, it is also possible he left Valve given a certain angst about the lack of releases from Valve as reported by Eurogamer.
It is unclear what Erik will be working on, now that he has returned to Valve but I'm sure many people (including me) are hoping for that ever elusive Half-Life sequel.