Clockwork Rumbler

Momentum Based. The Rumbler's motion is entirely based on momentum. It can only modify its movement by its walking speed each turn, or by redirecting its momentum through use of its grapple/slingshot. Whenever the Rumbler moves through the space of a creature, It makes a Slam attack against that creature.

Knockback modifies the current trajectory of the Rumbler, and any effect that would reduce its movement speed to zero Halves it current speed instead. Gaining elevation reduces The Rumbler's speed by 5 per 10 ft. gained, loosing elevation increases the Rumbler's speed by 5 ft. for every 10 ft. lost

After every 2 turns of combat, the Rumbler moves its current speed in its current direction of travel

Running into an Object deals 2d6 bludgeoning damage plus 1d6 for every 5 movement speed above 30 to the object. If the object is not destroyed, The Grappler's bounces off at half its previous speed. If it is destroyed, The Grappler continues through the rubble unimpeded.

Conductive Body. Whenever the Rumbler is subjected to lightning damage, it takes no damage. Instead, its weapon attacks deal an extra 11 (2d10) lightning damage until the end of its next turn. Also until the end of the Rumbler's next turn, any creature that touches it or hits it with a melee attack while within 5 feet of it takes 11 (2d10) lightning damage.

Immutable Form. The Rumbler is immune to any spell or effect that would alter its form.

Heavy Armor. The Rumbler counts as Huge creature wearing heavy armor wielding a heavy weapon for all knockback interactions.

Magic Weapons. The Rumbler's weapon attacks are magical.

Slam. Melee Weapon Attack: +8 to hit, reach 0 ft., one target. Hit: 15 (2d6 + 8) bludgeoning damage, increased by 1d6 for every 5 movement speed above 30. If the target is Large or smaller, it must succeed on a DC 18 strength saving throw or be knocked prone. If the strength save is successful, the Rumbler's current speed is reduced by 10. 

Charge. The Rumbler looses 20 ft. of movement speed and charges its Conductive Body trait.

Clothesline. A targeted Creature must succeed on a DC 14 DEX saving throw or take 1d6 slashing damage for every 10 speed of the cable segment that hits them and be knocked prone.

Unresponsive. The Clockwork Rumbler can take no reactions

The Rumbler can take 6 legendary actions, choosing from the options below. Only one legendary action option can be used at a time and only at the end of another creature’s turn. The Rumbler regains spent legendary actions at the start of their turn.

Grapple. The Rumbler shoots a grappling cable at an object between 15 and 60 feet away. If the target is a solid object of sufficient strength (DM discretion), The Rumbler's path becomes an arc around the grappled point. The grapple can be released at any point. A creature In the path of the cable is targeted by Clothesline. The cable has an AC of 16 and 25 HP, which are not a part of The Rumbler's main HP pool.

Slingshot (Costs 2 Actions). The Rumbler shoots 2 cables at different objects between 15 and 60 feet away. If the targets are solid objects of sufficient strength (DM discretion), the pair of cables can be used to spring The Rumbler back along its original path. A creature In the path of the cable is targeted by Clothesline. The cables have an AC of 16 and 25 HP, which are not a part of The Rumbler's main HP pool.

Accelerate. The Rumbler increases its speed in its current direction of travel by 10.