Simultaneously under contradictory Seeming and Disguise Self

Question

Before entering a fortified hobgoblin camp, the party bard cast Seeming on the whole group to make them all appear as hobgoblins, including herself.

Later, but still within the eight hour duration of the Seeming, she wanted to use her Hat of Disguise to change her own appearance away from that of a hobgoblin. Hat of Disguise allows her to cast Disguise Self at will. Seeming is not a concentration spell and at the time I may have believed that it made no provision for the caster ending it [my error, as pointed out by Wyrmwood]. Thus there was an apparent conflict between the effects of the Seeming vs. that of the Disguise Self. For the purposes of answering this question according to RAW, we can assume that the bard chose not to end the Seeming because she didn't want to end the effect on the rest of the party, just on herself.

The PHB rule for "Combining Magic Effects" says

The effects of different spells add together while the durations of those spells overlap. The effects of the same spell cast multiple times don’t combine, however. Instead, the most potent effect—such as the highest bonus—from those castings applies while their durations overlap.

Seeming and Disguise Self are not the same spell cast multiple times and thus we should 'add their effects together'. But since one effect is attempting to make the bard look like a hobgoblin and another is attempting to make her look not like a hobgoblin, it is difficult to determine how to 'add these effects together'.

Even though Seeming and Disguise Self are not the same spell, the descriptions of the spell effects themselves are nearly identical: Seeming says it

allows you to change the appearance of any number of creatures that you can see within range. You give each target you choose a new, illusory appearance...The spell disguises physical appearance as well as clothing, armor, weapons, and equipment. You can make each creature seem 1 foot shorter or taller and appear thin, fat, or in between. You can't change a target's body type, so you must choose a form that has the same basic arrangement of limbs. Otherwise, the extent of the illusion is up to you.

Disguise Self says

You make yourself--including your clothing, armor, weapons, and other belongings on your person--look different until the spell ends or until you use your action to dismiss it. You can seem 1 foot shorter or taller and can appear thin, fat, or in between. You can't change your body type, so you must adopt a form that has the same basic arrangement of limbs. Otherwise, the extent of the illusion is up to you.

At the time, as DM, I ruled, 'if I can't tell any functional difference between these spell effects, I am going to treat them as if they were the same spell effect. Since there is no clear guidance on what 'most potent' means, I will then apply the errata rule for combining magical effects that the more recently-cast takes precedence.' I allowed the bard to change her appearance to what she wanted with the Disguise Self.

I am comfortable with my decision as a DM, but I'm wondering if there is any RAW guidance for when there are two non-identical spells with contradictory effects that are to be 'added together'?

Answer 1

I would challenge the frame of the question, as you can indeed dismiss the spell sooner.

Because your assertion

Seeming is not a concentration spell and makes no provision for the caster ending it

contradicts

The spell lasts for the duration, unless you use your action to dismiss it sooner. Seeming, description, paragraph 2, last sentence.

However, for the more general case, if the spells' descriptions do not specifically call out canceling of each other, you could rule the higher spell level takes precedence, as mentioned in the answer you linked, or even come up with a diminished effect of the more potent spell.

If you wanted it to be less deterministic, you could convert it to a check. Perhaps you could use counterspell for guidance, something like a contest, where each rolled a spellcasting ability check plus the spell level. In your example, the bard would be rolling both checks.

I can certainly see how your bard might not have wanted to dismiss the spell, since it was in effect on the whole party. It might even be fun to do something like the "2 weeks" scene from Total Recall (or "3 days" from the remake).

From the DMG

The D&D rules help you and the other players have a good time, but the rules aren't in charge. You're the DM, and you are in charge of the game. That said, your goal isn't to slaughter the adventurers but to create a campaign world that revolves around their actions and decisions, and to keep your players coming back for more! If you're lucky, the events of your campaign will echo in the memories of your players long after the final game session is concluded.