The presence of strong evil registers on your senses like a noxious odor, and powerful good rings like heavenly music in your ears. As an action, you can open your awareness to detect such forces. Until the end of your next turn, you know the location of any celestial, fiend, or undead within 60 feet of you that is not behind total cover. You know the type (celestial, fiend, or undead) of any being whose presence you sense, but not its identity (the vampire Count Strahd von Zarovich, for instance). Within the same radius, you also detect the presence of any place or object that has been consecrated or desecrated, as with the hallow spell. You can use this feature a number of times equal to 1 + your Charisma modifier. When you finish a long rest, you regain all expended uses.
PHB p.196: A target has total cover if it is completely concealed by an obstacle.
A wall therefore gives total cover, even if it's just a thin interior divider wall. Does this mean the Count above would not be sensed by a paladin if he was inside a castle, even if only 5 feet away behind a wall?
The "behind total cover" bit really seems to nerf this ability. It seems the paladin would only sense evil if it were pretty obvious anyway (i.e. not behind total cover). Or am I missing something here?
It sees through disguises and magical shapeshifting abilities (and possibly possession, depending on the DM). It's not an ability a Paladin should be able to spam just to find something to kill (like ye olde "Detect Evil" from days of yore)
He/she can only do it 1+Cha mod times before a rest, so not a lot of spam there. I'm thinking of the situation where a paladin is on one side of a door/wall and a dozen undead are on the other; going by this rule, it seems the paladin wouldn't know any more about them being there than any other character.
Which is true, in some situations, I suppose. It is a semi-silly ability, and certainly situational. In which case, read the first sentence of my first post 
El_Condoro wrote:
I think it's real utility comes from detecting relevant beings that are invisible, ethereal, stealthy, shapechanged, polymorphed, disguised, etc.
Clutchbone wrote:
El_Condoro wrote: