What Traits are determined by the Summon Aberration Spell?

Question

For reference, in a session last night the Warlock of the group had a summoned Beholderkin from the Summon Aberration Spell. A Lair action occurred which could have caused the Beholderkin to have been rendered Prone. I as the DM know that generally Beholderkin are immune to the Prone status, however it does not specifically state in the Summon Aberration spell that you use the stat block in the source book (i.e. Monster Manual) as it provides it own.

For reference the spell states:

You call forth an aberrant spirit. It manifests in an unoccupied space that you can see within range. This corporeal form uses the Aberrant Spirit stat block. When you cast the spell, choose Beholderkin, Slaad, or Star Spawn. The creature resembles an aberration of that kind, which determines certain traits in its stat block. The creature disappears when it drops to 0 hit points or when the spell ends.

The spell also provides a stat block to use.

So, which traits are used from the Monster Manual (or other source book) stat block in conjunction with the Summon Aberration stat block?

Answer 1

The spell description and associated stat block have everything you need to know.

The rules for Spellcasting state:

Each spell description in Chapter 11 begins with a block of information, including the spell’s name, level, school of magic, casting time, range, components, and duration. The rest of a spell entry describes the spell’s effect.

Here, the rules are telling us that everything you need to know about a spell’s effect is contained in its description. Since the provided stat block does not list any condition immunities, and the description doesn’t direct us anywhere else for information, the summoned creature has no condition immunities.

When the spell description says, "determines certain traits in its stat block", it is referring to those traits that the provided stat block assign to one form only, such as Regeneration:

Regeneration (Slaad Only). The aberration regains 5 hit points at the start of its turn if it has at least 1 hit point.

Spells that require you to look elsewhere for information will tell you that. For example, create homunculus states:

The statistics of the homunculus are in the Monster Manual.

Instead of providing a stat block for the homunculus, the spell description directs you to where it can be found. Since the description for summon aberration does not direct us to the Monster Manual like we see in create homunculus, we do not need to consult the Monster Manual to figure out how summon aberration works.

Answer 2

It's not a beholderkin (or at least one like in the Monster Manual)

Thomas Markov's excellent answer is a clear dissection of the rules involved in understanding why the summoned creature does not have traits from the MM.

This answer just attempts to add some narrative justification.

While it is tempting to say 'I know that beholderkin are immune to prone', you are correct in your question that the summoned creature only resembles an actual beholderkin. The power of the spell shapes energy into a form that approximates a beholderkin, but it is not actually one.

Or, if you prefer, the spell summons the soul of an actual beholderkin but places it inside a body that spell makes, but one that is fundamentally different from the one that a beholderkin would actually have, and is more limited.

The easiest way to see this, I think, is to realize that the spell creates more and more powerful bodies as it is cast at higher levels (AC and hp increase), but still not approaching the power of the actual aberration (about half the hp of the slaad, for example). In the case of the Slaad, the spell does not specify the color of the Slaad it summons, which is so fundamental to understanding how the creature would act that it is clear you are not getting a 'real' Slaad with it.

Thus, the summoned beholderkin simply does not have the immunity to prone that a MM-statted beholderkin would have because it is not actually a beholderkin.

Answer 3

Thomas Markov's answer is correct for RAW. However, I want to add an additional perspective regarding your instinct that maybe the Beholderkin should be immune to the prone condition:

If it feels correct for your table, that would be a perfectly legal, reasonable and cool house ruling. By design, DnD explicitly and specifically gives the DM the ability to make house rules about cases like this to better make the game fit the players. This ruling fits thematically, it makes reasonable sense, and it's not game-breaking in any important way. Even though it likely won't make a huge difference either way, it gives the Warlock player a moment of feeling cool/powerful, which is generally worth it.

The only important point here is that you're not obligated to make that ruling if you don't think it fits well within your game - you're only really obligated to follow the stat block as written. As discussed in other answers, the stat block provided does not give any indication that the Beholderkin would be immune to the effect. Likewise, players should not reasonably expect that it would be immune unless they've discussed this specifically with you ahead of time.