oh noes! ok...description now? I had a go above
The arm shedding needs to be sorted out first, so let's have a stab at that.
Shed Arms. Whenever the giant brittle star takes at least ## damage at one time from an attack that required a Melee or Ranged Attack roll or if the brittle star is grappled by a creature that is Large size and/or possesses a Strength of ## or higher, roll a d## to determine what happens.
01-##: Nothing else happens.
##-##: The attacker makes a DC ## check (Strength for weapon hits, spellcasting ability for magic hits). If they succeed, one arm is severed from the brittle star if it has any arms left.
##-##: One arm is severed from the brittle star if it has any arms left, but the brittle star takes half damage from the attack that severed the arm.
In addition, if a giant brittle star is threatened or attacked by a creature larger than itself, or its hit points are reduced to ## of fewer, it can attempt to deliberately tear off an arm. The brittle star makes a DC ## Strength check and severs one of its own arms if it succeeds.
A
severed arm flops and writhes about for #d# rounds as it dies. The arm is unable to attack or grapple and has a speed of 5 feet. The brittle star has no control over its severed arms.
A giant brittle star's speed is reduced to 15 ft. if it loses one or two arms, 10 ft. if it loses three and 5 feet if it loses four. If it loses all five arms it has a speed of 0 feet.
If the brittle star finishes a long rest its arm stumps regrows enough to remove one "arm's worth" of speed reduction, so it has speed 20 ft. with one lost arms, 15 ft. with two or three lost arms, 10 ft. with four lost arms, and 5 ft. if is has lost all five arms. The brittle star regains its speed and arm attacks when its
Regrowth trait replaces the lost arms.
Option: A giant brittle star with a shortened, partially regrown arm might be able to attack with it, but the arm attack will have a 5 ft. reach or 10 ft. reach instead of the 15 ft. reach of a full length arm.