Prerequisite: To gain access to this spell a creature must normally learn undercommon and gain favour with Blibdoolpoolp, goddess of the Kuo-toa.
By speaking words of devotion in undercommon and holding your arm aloft, it is magically transformed into a glorious chitinous claw until the spell ends. The claw counts as a simple natural melee weapon with which you are proficient, dealing 3d8 bludegoning damage on a hit. In addition, once per turn when you hit a target with the claw, you may attempt to Grapple it. While grappling a target with the claw you may not use it to attack any other target.
You may cast this spell upon yourself a second time in order to gain a second claw. When you attack with one claw using the Attack action on your turn, you can immediately make one attack with the other claw as a bonus action.
At Higher Levels. When you cast this spell using a 5th- or 6th-level spell slot, the damage increases to 4d8. When you cast it using a spell slot of 7th level or higher, the damage increases to 5d8.
This spell is my attempt at the signature spell of Blibdoolpoolp, the goddess of the Kuo-toa.
Basically it's modelled after shadow blade except that it must be cast at a minimum of 3rd-level due to the longer duration and lack of concentration required (as the spell is fully transforming the body part). The claw deals a more resisted damage type, lacks the finesse, light and thrown properties (meaning it requires Strength) and it has no mechanic to gain advantage. Instead it has the ability to Grapple a target, and you can make a bonus action attack if you cast it upon yourself twice.
Strictly speaking this spell should only be granted to worshippers of Blibdoolpoolp, who would most likely be Death Domain Clerics, however it's also a fun twist on shadow blade which Clerics and Druids can't normally take, so your DM is of course free to waive this recommendation (and the requirement to know undercommon).