Hurl through Hell (HtH) is the level 14 Otherworldly Patron feature for a Fiend Warlock. The tarrasque is not resistant or immune to psychic damage, is not a fiend, and is capable of being teleported to another plane. HtH doesn’t cause a saving throw, is not a ranged spell attack, and works only if the attack hits. I just want someone to confirm whether the tarrasque will be affected by this feature.
Yes, the Tarrasque can be affected by Hurl Through Hell
I think you answered this question yourself, but just to confirm for you, Hurl Through Hell states:
Starting at 14th level, when you hit a creature with an attack, you can use this feature to instantly transport that target through the lower planes. The creature disappears and hurtles through a nightmare landscape.
At the end of your next turn, the target returns to the space it previously occupied, or the nearest unoccupied space. If the target is not a fiend, it takes 10d10 psychic damage as it reels from its horrific experience.
The only requirements for using Hurl Through Hell is to hit the creature with an attack, which the Tarrasque does not have any immunities or resistances to (beyond a high 25 AC).
To take the damage the creature must not be a fiend or immune to psychic damage and as you already noted, the tarrasque is a monstrosity without any resistances to psychic damage.
The only source of confusion I could see is the tarrasque's Magic Resistance but as you stated this ability does not provide or saving throw, nor is it a line spell or ranged attack spell that might be reflected by Reflective Carapace.