Inspired by this question: What good is the paladin's Divine Sense?
I am running Lost Mines of Phandelver to introduce a couple of people to D&D (just me and my two players; I use DMPCs to pad out the party), and one of my players is a Paladin. During their time in Thundertree, he used Divine Sense a few times to try to detect the zombies throughout the town (there's a sign that says "Beware! Zombies" or something, so this was a completely in-character thing to do).
Since, as the linked question mentions, it's a very niche ability in D&D 5e, I wasn't that familiar with it; I remembered that it detects undead and fiends (forgot celestials at the time, but it's irrelevant to this case) within 60 feet, but forgot about the total cover thing, so we proceeded without the total cover part of the ability. It was very useful for sniffing out those zombies hiding in the buildings.
Only later (because of a question on here, I think – this was a while ago), I realised that total cover was part of how it worked, and informed my player that, what happened happened, but going forward we'd have to bear total cover in mind. His response was basically "ok, you're the DM, but doesn't that kinda make this ability useless?"; my only response was about detecting invisible fiends or undead, which he accepted gracefully, but I doubt we'll be seeing much more use out of it…
They're now in Wave Echo Cave (our sessions are quite infrequent), but have not encountered anything undead yet, and will eventually progress onto Out of the Abyss, which has many fiends, and in light of the question I link to at the top, I am starting to wonder whether I should go back on myself and allow my player's Paladin to use Divine Sense like how he used it in Thundertree. I think it will be more fun if he can actually use his ability outside of its otherwise rare niche uses.
This question is about the impacts on balance. What issues would arise from me ruling to ignore the total cover rule of Divine Sense? Would the ability become overpowered or otherwise unbalanced compared to other classes?
Best Answer
If you have doubts about balance, keep in mind that ultimately that depends on how you as a DM build the campaign and possible encounters where this ability could be used.
If you allow your player to by-pass total cover and then you consider that some parts of the adventure would be "ruined" by that, you're free to change it a bit to avoid that problem.
If you want to buff the ability a little bit without totally by-passing the total cover restriction, you could let him "sense" that there are indeed undead, celestials or fiends in range but not the precise location of them due to the total cover.
Either way, I'd say that it wouldn't break the class unless your campaign uses hidden undeads as a key part of the plot.