Dyrrn’s Tentacle Whip is from Eberron: Rising from the Last War , pg. 276
The main question is does the tentacle count as a appendage of the player for the purposes of making a melee spell attack.
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The Whip appears in the Martial Melee Weapons table of the PHB. Dyrnn's tentacle whip says:
Weapon (whip), very rare (requires attunement)
So Dyrnn's Tentacle Whip is a melee weapon. When attacking with a melee weapon, you make a melee weapon attack, not a melee spell attack.
The Reach property of weapons says:
This weapon adds 5 feet to your reach when you attack with it
If you are casting a spell that calls for a melee spell attack, you are not attacking with Dyrnn's Tentacle Whip so the Reach property of the weapon does not apply.
Of course, a DM could rule that it does count, and from a flavor perspective this seems reasonable, but that would make this magic item extremely powerful, far more powerful than it is otherwise.