When do PCs get spot checks for a gelatinous cube in poor light?

Question

I'm new to DMing, and trying to figure out what's fair vs. fun on when the player characters have a chance to spot a gelatinous cube. So if they are in a dimly lit dungeon, when do you have the PCs roll the unmodified DC 15 spot check?

The 3.5e Monster Manual says:

Transparent (Ex): Gelatinous cubes are hard to see, even under ideal conditions, and it takes a DC 15 Spot check to notice one. Creatures who fail to notice a cube and walk into it are automatically engulfed.

In our dungeon, I had the gel.cube hiding around a corner and the PCs had to make a spot check when they rounded the corner. The guy in front made the spot and stopped everybody from just blindly walking into it and being auto-engulfed (the 3.5e rules).

To simplify the question though, let's say the PCs are walking down a 130 foot dark passageway with the gel.cube at the far end. Does the Dwarf get a spot check at 60 feet (60' max darkvision), then later a spot check for the elf when dim torchlight reaches it (I read in the 3.5 PHB pg. 16 Elves get twice the dim light vision distance as humans), then at 20-30 feet when the human/halfing is within decent torch range? Or is it so hard to spot at that distance (increasing difficulty by +4 or +8 or more on the DC due to distance and a nearly invisible unmoving gelatinous cube), that they'd have to wait till they are much closer? Or do you allow multiple spot checks per character as they get closer? My thought is wait till much closer, but what is fair and fun?

---

Related questions:

• This thread below talks about it vs. darkvision in 5e terms. I'm playing 3.5e, so I might go with a +4 on the DC if they try to spot it at medium range on their vision:
  Is it harder to see a Gelatinous Cube with darkvision?
• This thread makes it clear that it isn't truly invisible:
  When do you see a gelatinous cube?
• I feel a second question coming up about how many spot checks can you make as you approach something hard to see from far away, but that's bending the rules on asking questions so I'll leave this post with this link which might answer that question for me as I go read it:
  How to use spot checks in a wilderness encounter?

Answer 1

Roll everybody all at once, determine encounter distance

The party, being inexperienced, says "we walk down the corridor and round the corner". You say: "Okay, roll Spot". Everybody rolls. Alternatively, you roll spot secretly for them. Either way works, depending on your playstyle and how exactly the PCs are advancing. If the characters are sneaking and have protection from the cube's blindsight (e.g. tower shields or Hide in Plain Sight) also roll Spot (at -5) for the cube.

The SRD puts the penalty due to distance at -1 per 10 feet. Something has to be within your sight range for you to even get a check unless there's something special going on where the DM should be ignoring this often-problematic set of rules (in this case-- a monster without a light source in a dungeon-- there isn't).

The corridor is dark, but the PCs carry a torch. This is necessary for the humans and elves to be making checks.

For each character that succeeds at the DC 15 check, calculate the distance at which they would notice the cube. Also calculate the distance at which the cube notices the players (60' typically).

• The dwarf sees the Cube at 5'+ (their check result - 15)×10', to a maximum of 60' with a total of 21 on the Spot check.
• The elves see the Cube at 5'+ (their check result - 15)×10', to a maximum of 80' with a total of 23 on the Spot check-- a torch has a dim light radius of 40'.
• The humans see the Cube at 5'+ (their check result - 15)×10', to a maximum of 40' with a total of 19 on the Spot check.
• The cube sees the party at 60' if they aren't hidden from it, or 10'×(its Spot check - the party's lowest Hide check) to a maximum of 60' if they are sneaking but the cube somehow succeeds on the opposed Spot vs. Hide check. (Noticing that something is up via Listen vs. Move Silently works similarly, but is outside of scope here).

If no one saw the cube, the front of the party in the marching order walks right into it (no save) and is cube-food. The others notice this, the cube rolls its initiative and gets its surprise round before anybody else gets to roll initiative (unless it was somehow also surprised), and then the party rolls initiative and normal combat starts.

If anyone saw the cube, stop the party where that character or characters who saw the cube from the furthest away see the cube and describe what they saw. The other characters can repeat their checks if they like, but it might not be possible for them to succeed if there's not enough light for their senses.

If the cube isn't right up against the corner and thus able to 'see' around it, it may not be able to see the party until they round the corner. If that's the case, and it's within 60' of the corner, use its distance to the corner as its max sight distance rather than 60'. That same max distance will also apply to the party for those members whose max distance is greater than the distance from cube to corner.