As soon as I made Derkeethus a blade, he ran away and never came back. I have tried the player.placeatme cheat code for Derkeethus after I made him a blade but as soon as he spawns (in his blades armour) he leaves again and is never seen in Sky Haven temple again. Why does this happen? How can I fix it?
UESP's article on Derkeethus notes:
I would guess that this is probably due to a bug in his AI data, and indeed the notes for the 1.05 Beta version of the Unofficial Skyrim Patch indicate that this as an AI problem that it fixes:
... so installing the latest version of the USKP should fix this problem, if you're on PC.
If you're on a console, then I'm afraid you're out of luck. Best you can do is drop by Darkwater Crossing every once in a while to chat with Derkeethus the Blade and see what a good a job he's doing keeping the little mining outpost dragon-free.