Have been playing a Fathomless Warlock recently, and have found the way the tentacles function outside of attacking and moving it is somewhat vague. If I use the tentacle to attack someone who doesn't know I'm there, will they become aware of my warlock? I assume this is up to the DM but my DM wanted to try and reach out first.
RAW Revealed, but that doesn't make a lot of sense.
Assuming you have succesfully hidden, this should work! Fathomless Tentacle only says that
... you can make a melee spell attack against a creature.
Hiding rules suggest that the way to become visible is:
you give away your position if you make noise, such as shouting a warning or knocking over a vase.
There is no verbal command or anything that suggests that moving/directing to attack does anything physical that would result in losing your Hidden state.
A DM could decide that such a command does make you visible
Hiding in 5e is very much dependent upon DM rulings, with the Basic Rules even stating:
The DM decides when circumstances are appropriate for hiding.
However, given my statement above, it seems that the Fathomless Tentacle shouldn't do anything that would prevent you from maintaining your Hidden status.
Who is actually attacking?
As SDJZ says in their answer to a similar question about spiritual weapon, the RAW does state that the caster is actually the attacker, and that would make one visible.
However, a DM ruling that while the caster is 'attacking', it is the Fathomless Tentacle that is interacting at the hidden bequest of the caster and thus remain hidden.
But RAW is definitely losing Hidden status and then DM would then have to rule whether or not you could Hide again after revealing your position.
When you create the tentacle, you can make a melee spell attack against a creature ...
And from the rules on Unseen Attackers and Targets:
If you are hidden--both unseen and unheard--when you make an attack, you give away your location when the attack hits or misses.
You (not the tentacle) are making an attack, therefore you give away your location.