At first I wanted to record a video that I got glithed and was placed outside of the map.
And suddenly we've noticed the Stalker in game. We were quite surprised by this, we were thinking it's another polayer hacking or something.
Apparetnly it is a NPC showing in the game from time to time.
Appears on these pages
For more details, please refer to the WARFRAME Wiki Media Policy. The Stalker in the Codex