Does transparent cover negate paladin Divine Sense?

Question

Some background: I want to have the bad guy necromancer have a trap in front of the secret entrance to his lair. He has a small frozen lake that abuts the entrance. To invade, the players would have to walk across delicate ice. Under the ice, the necromancer has concealed a number of skeleton defenders. The entrance is entrapped to activate sliding blocks that will break the ice, giving the characters terrible footing and surrounding them with a horde of enemies.

The problem is that I have a Paladin in the group with Divine Sense:

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.

I would like to make the ice translucent, so that the PCs can sense that something is down there with a good Perception roll (The skeletons would have the Perception Roll at advantage, due to the PCs being backlit). Would this negate the Divine Sense? Does transparent cover count as cover for this ability? The alternative is to just make the ice opaque, but that would reduce the Creepy Factor™.

The answer to this question has further ramifications: Does DS work through a sheet of thick glass, for instance? Or in a a mirror held around a corner? Could a vampire hide his nature just by standing on the other side of a window?

My first instinct is to say that cover is a game term for something that blocks your ability to target the enemy with an attack, and thus the DS would not work, but if anyone has good compelling arguments one way or another, it might forestall distracting table debate.

Answer 1

Divine Sense can't target something behind total cover.

Your instincts are correct.

The rules on cover state:

Walls, trees, creatures, and other obstacles can provide cover during combat, making a target more difficult to harm. A target can benefit from cover only when an attack or other effect originates on the opposite side of the cover.

There are three degrees of cover. [...]

[...]

A target with total cover can't be targeted directly by an attack or a spell, although some spells can reach such a target by including it in an area of effect. A target has total cover if it is completely concealed by an obstacle.

The usage of the word "concealed" here is problematic, but I think the usage here is assuming you have a solid, non-transparent object providing your cover. I think the intent is clear – if an attack or effect would be blocked completely, then the target is behind total cover.

Since the paladin's Divine Sense feature cannot detect through total cover, I think you are in the clear. The sheet of ice acts much like a wall of force, except the ice can be damaged or broken through, and I'd rule that wall of force provides total cover to enemies on the other side, as it "makes the target more difficult to harm."

This ruling is further reinforced by an unofficial tweet by rules designer Jeremy Crawford from April 2016, in response to a question about whether wall of force provides cover:

Cover is a physical obstruction, not necessarily a visual one.

This also serves to show that the use of the word "concealed" in the description of total cover was in error.

Answer 2

Since this still gets cited and pops up on searches, I figured I'd throw in my two cents for people still coming by.

To preface, I think it's important to consider that Divine Sense is not visual, but functions more as a 6th sense.

TL;DR: glass as thick as most windows wouldn't offer protection against a crossbow bolt and therefore shouldn't contribute to cover and block Divine Sense. The mirror and thick glass is up to you.

As LegendaryDude pointed out, a thick ice sheet almost certainly qualifies for total cover, but most glass (let's imagine a glass panel with window-thickness) wouldn't qualify for much the same reason as standing behind a tapestry wouldn't. That is, we have hidden (unseen in particular), covered, or both. The former cam impose disadvantage on ranged attack rolls, while the latter can increase AC against them, so a practical test (RAI, in my opinion) is considering how the obstacle in question would affect an attacker with a heavy crossbow:

A large bush or a fog cloud wouldn't provide protection against a bolt, but the target would be hidden (unseen) and the attack would be at disadvantage. A large tree trunk would stop the bolt and thus provide cover--it could be used to hide at the DM's discretion, however. The glass panel would neither hide nor protect (cover) the target from a heavy crossbow bolt.

Based on this, Divine Sense shouldn't be blocked by such a panel of glass.

For the other situations, I'd reason that the mirror wouldn't work, since you're just seeing a reflection of the creature and Divine Sense doesn't rely on vision (e.g. fog wouldn't stop it), and that any material thick enough to stop a heavy crossbow bolt, even glass, would do the trick.

We can also get into weird territory if we start ruling that an average window can block Divine Sense. Granted, held items are treated differently, but it starts to seem reasonable that enough costume material or armor would be enough to block it, and that's clearly not the case.