Amphibious. The land star can breathe air and water.

Capable Clambering. A land star ignores difficult terrain penalties caused by slippery or smooth surfaces and can climb difficult surfaces, including upside down on ceilings, without needing to make an ability check.

Fission. During their reproductive season, a land star can split in half to become two new land stars; this slow process takes 10 to 20 (2d6 + 8) days but does not inconvenience the creature. The two new land stars each have 21 (2d10+10) hit points and three arms. As they gain new arms with their Regrowth ability, their hit points slowly increase until they reach 42 (5d10 + 15) hit points when they grow six full length arms and a complete body-disc. It usually takes at least three months (6d10+60 days) for a split half to regenerate into a whole walking giant brittlestar.

Regrowth. If a land star loses an arm, organ or other body part and survives, it regrows the lost body parts as it heals. It takes 15 (1d10 + 10) days for a land star to replace a missing arm.

Rudimentary Vision. The land star has disadvantage on Wisdom (Perception) checks that rely on sight. It perceives patterns of light and darkness and can only see the silhouette or shadow of a creature or object; it can not see colors or fine details.

Shed Arms. Whenever the land star takes at least 7 damage at one time from an attack that required a Melee or Ranged Attack roll, or if the land star is grappled by a creature that is Large size and/or possesses a Strength of 18 or higher, roll a d6 to determine what happens.

In addition, a land star can choose to deliberately tear off one of its own arms as a reaction (see Auto Dismemberment).
 If a land star loses an arm to a grapple attack or self mutilation, the land star takes 4 (1d4 + 2) damage or the damage rolled by the attack, whichever is higher.
 A severed arm flops and writhes about for 1d10 rounds as it dies. The arm is unable to attack or grapple and has a speed of 5 feet. The land star has no control over its severed arms.
 A land star's speed is lowered by 5 ft. if it loses one or two arms and halved if it loses three or four. If it loses five arms its speed becomes 5 feet and if it loses all six it has a speed of 0 feet.
 If the land star finishes a long rest its arm stumps regrow enough to remove one "arm's worth" of speed reduction, so it is lowered by 5 ft. with two or three arms, halved with four or five lost arms, and reduced to 5 ft. if it has lost all six arms. The land star regains its speed and arm attacks when its Regrowth trait replaces the lost arms.

Option: A land 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.

Multiattack. The land star can make up to three attacks against different targets: it can make one attack with its bite against a target it is grappling, the remaining attacks must be arm attacks. If the land star has fewer than three arms because of its Shed Arms ability, the maximum number of arm attacks it can make equals its current number of arms.

Arm. Melee Weapon Attack: +4 to hit, reach 15 ft., one target. Hit: 4 (1d4 + 2) bludgeoning damage. If the target is a creature, it is grappled  (escape DC 14; with disadvantage on escape checks).  Until this grapple ends, the target is restrained, and the land star can't use that arm to attack a different target. If the arm hits a target that is already grappled, the target must succeed at a DC 14 Strength check or be pulled within reach of the land star's bite attack, or into the nearest empty space if that isn't possible.

The land star has six arms (unless it loses some due to its Shed Arm trait), each of which can grapple one target. 

Bite. Can only attack grappled targets. Melee Weapon Attack: +4 to hit , reach 5 ft., one target. Hit  4 (1d4 + 2) bludgeoning damage plus 4 (1d4 + 2) slashing damage.

Auto Dismemberment. A land star can automatically lose an arm (see Shed Arms) as a reaction in order to escape a dangerous situation. For example, if the ophiuroid is being attacked by a creature larger than itself or has its hit points reduced to 20 of fewer.