The thing is, I definitely remember us finding "u can not teleport near it" radioactive crystals at some point.
Maybe it was on the route to that Beholder's lair, when Viserys and Co went clearing out a trade route for Xorn...
I still want to loot useable amounts of Blightburn, btw.
It is the top-tier sort of last-ditch protection for any of our installations.
Just pop a lead case open, and that overpowered divine Avatar you just accidentaly'd into reality can't just teleport away.
Let alone thigs lesser, like, say, a Pit Fiend.