How does the spell Black Tentacles work?

I have trouble interpreting how exactly the spell "Black Tentacles" works. Is the following sequence correct:

1. The caster casts the spell and chooses the area
2. For every creature in the area, the caster makes a spell attack roll against their Fortitude DC. On a success, that creature takes 3d6 damage, double that amount on a critical hit. The 3d6 are rolled separately for each creature that is hit.
3. Creatures that where hit are grabbed

Later, a creature that is grabbed takes its turn.

1. It tries to escape, succeeding on the third try (though unlikely, we just had that happen)
2. The creature ends its turn in the area. The spell text says "Whenever a creature ends its turn in the area, the tentacles attempt to grab that creature if they haven't already"
3. Does the caster now make another spell attack against the creature's Fortitude DC, grabbing the creature and dealing 3d6 damage again (or double that on a crit)
4. The creature does not take 1d6 damage it was not already grappled when the turn ended

My main question is about the step 3 in the creature's turn, I am not sure what the "the tentacles attempt to grab that creature" means exactly, is that simply a reference to the first part of the spell?

You make another spell attack roll against their Fortitude DC

The spell says:

Make spell attack rolls against the Fortitude DC of each creature in the area. Any creature you succeed against is grabbed by a tentacle and takes 3d6 bludgeoning damage. Whenever a creature ends its turn in the area, the tentacles attempt to grab that creature if they haven't already, and they deal 1d6 bludgeoning damage to any creature already grabbed.

The tentacles attempting to grab a creature that ends its turn in the area is an attack by the tentacles, resolved in the way explained by the first sentence cited above. The spell has no memory and does not care if a creature that ends its turn in the area was grabbed by it before or not.

While a creature remains grabbed, they only suffer 1d6 on each subsequent turn. If the creature manages to free itself, it can try to leave the area where the tentacles are, to avoid being attacked by the tentacles again at the end of its turn and suffering the 3d6 from the attack again if it hits.

I think the term "if they haven't already" serves to clarify that the tentacles cannot attack a creature that they are already grabbing, and do not deal attack damage on top of ongoing damage. (Agreeing with Anne).

• So you would also say that my interpretation that the tentacles can critically hit is correct? Because other spells seem to explicitly state they deal double damage on a crit, while this one does not. Commented Aug 4, 2022 at 9:02
• The rule for critical hits states "If you critically succeed at a Strike, your attack deals double damage", and as I understand it, a Strike means "You attack with a weapon you're wielding or with an unarmed attack", so no, a critical roll would not double the damage as the spell attack is not a weapon attack and therefore no strike. Commented Aug 4, 2022 at 9:18

I think Groody's answer is correct, but I want to address the ambiguity in this spell's description.

Oily black tentacles rise up and attempt to capture creatures in the area. Make spell attack rolls against the Fortitude DC of each creature in the area. Any creature you succeed against is grabbed by a tentacle and takes 3d6 bludgeoning damage. Whenever a creature ends its turn in the area, the tentacles attempt to grab that creature if they haven't already, and they deal 1d6 bludgeoning damage to any creature already grabbed. (emphasis mine).

There are multiple ways to understand "if they haven't already".

1. "the tentacles attempt to grab that creature if they haven't already attempted that since the spell was cast": this seems like the most grammatically literal interpretation of the description, but it also makes the spell feel weird.
2. "the tentacles attempt to grab that creature if they haven't already managed to grab it and are still holding it": this isn't what the spell's description says, but this is the way I have seen it interpreted everywhere. It also makes sense that tentacles would try to grab any creature, without any consideration for what happened in the previous turns.
3. "the tentacles attempt to grab that creature if they haven't already managed to grab that creature, independently of whether this creature escaped or not since then.": this gets the worst of both worlds, as it both doesn't feel more natural than (1) and is not much less stretched than (2).

I personally would rule this spell as (2), but would completely understand someone being unhappy about this ruling.