The Sentinel Dark Armour power "Obscure Sustenance" boosts endurance recovery.
It's listed as "+x% endurance recovery rate for y seconds on self", the same as many other powers that accept endurance mod slots.
However, you can't you slot endurance mod enhancements.
Weirdly, some sites seem to believe you can: https://hcwiki.cityofheroes.dev/wiki/Dark_Armor#Obscure_Sustenance
I guess they've just assumed it's so obvious it must work, no-one's bothered to check if it actually works, which it doesn't. 🙂
(And yes, I've tried slotting directly myself in case it's a UI help issue, but it doesn't work.)
Could we please have this added? It would seem to make things more consistent with other similar powers.