Can you use the Eldritch Blast cantrip on the same turn as the UA Lurker in the Deep warlock's Grasp of the Deep feature?

Question

The Lurker in the Deep warlock patron (from UA: Sorcerer and Warlock) gets the Grasp of the Deep feature at 1st level. Part of the feature description states:

[...] As a bonus action, you create a 10-foot-long tentacle at a point you can see within 60 feet of you. [...]

When you create the tentacle, you can make a melee spell attack against a creature within 10 feet of it. On a hit, the target takes 1d8 cold or lightning damage (your choice when it takes the damage) and its speed is reduced by 10 feet until the start of your next turn. [...]

As a bonus action on your turn, you can move the tentacle up to 30 feet and repeat the attack.

So you use a bonus action to conjure it, then you can make a melee spell attack - but does this melee spell attack count as an action?

• If not, you can eldritch blast and also attack with the tentacles on the same turn, right?
• And if it does, can you just eldritch blast and then use the second hit as a bonus action?

(I know that that there is now an official version of this warlock patron published as The Fathomless in Tasha's Cauldron of Everything, but I liked this version way better.)

Answer 1

Yes, you can use the tentacle attack and eldritch blast on the same turn

The feature - which is almost exactly the same as the Tentacle of the Deeps feature from the final Fathomless patron detailed in Tasha’s Cauldron of Everything - says:

As a bonus action, you create a 10-foot-long tentacle at a point you can see within 60 feet of you. [...] When you create the tentacle, you can make a melee spell attack against a creature within 10 feet of it.

And later:

As a bonus action on your turn, you can move the tentacle up to 30 feet and repeat the attack.

In both cases, making the attack is part of the same action that either creates or moves it. This is pretty explicit in the move. In the case of the initial creation, thanks to the principle that there aren’t any hidden rules in 5E, and the absence of any general rule that making an attack must always be an Action, the language “when you create the tentacle” indicates the attack is made as part of the same bonus action used to create it.

(Remember that the Attack Action is just one possible action, available to everyone, which allows a character to make an attack. An “attack” is the separate process of making an attack roll, comparing it to AC, and rolling damage on a hit. There are numerous other rules and features that allow attacks as bonus actions, e.g. the offhand attack made as part of the Two-Weapon Fighting rules.)

So in this instance, the attack is made as part of the same bonus action used to create or move the tentacle. As the feature doesn’t involve casting a spell, it doesn’t interfere with the user’s ability to cast other spells on their turn. So a character can, on the same turn, summon or move the tentacle and make an attack with it as their bonus action, and cast a spell with a casting time of 1 action, including eldritch blast.

Answer 2

Attacking with the tentacle is always a bonus action.

The ability reads:

When you create the tentacle, you can make a melee spell attack against a creature within 10 feet of it.

So when we create the tentacle, we make a melee spell attack. To create the tentacle:

As a bonus action, you create a 10-foot-long tentacle

So with a bonus action we can create and attack with the tentacle. The feature goes on to say:

As a bonus action on your turn, you can move the tentacle up to 30 feet and repeat the attack.

So on subsequent turns, we can use our bonus action to move and attack with the tentacle.

Finally, since attacking with the tentacle is always a bonus action, we can use our action to cast eldritch blast.