Arcane Rubicon

Magic resistance: The Arcane elemental has advantage on saving throws and ability checks related to a spell in any way.

Innate Spellcasting. The Arcane elemental is a 14th-level spellcaster. Its spellcasting ability is Intelligence (spell save DC 17, +7 to hit with spell attacks). The Arcane elemental has 40 charges available and can use some to cast a spell, depending on the spell level. It can cast any spell of the specified level without components. Some of its actions use charges as well. The Arcane elemental can cast any 1st through 4th level spell as if it used a 5th level spell slot, spending only two additional charges.

Cantrips: no charge, cast as if the elemental were 13th level

1st or 2nd level: 1 charge

3rd or 4th level: 2 charges

5th or 6th level: 3 charges

7th or 8th level: 5 charges

9th level: 10 charges; cannot cast wish.

Arcane casting (7 spellcasting charges). The Arcane elemental uses one of the following abilities:

• Tear earth: In a 30-foot diameter around the Arcane Rubicon, the ground becomes difficult terrain for the rest of the round. Any creature that starts its turn in the area must make a DC 18 strength save or take 38 (4d10+18) bludgeoning damage and be knocked prone on a failed save, or half as much damage and not be knocked prone on a successful one
• Summon blaze: The Arcane Rubicon chooses a point within 60 feet of it. A 15-foot radius sphere of fire is created there and lasts until the end of the round. Any creature within that area must immediately make a DC 18 Dexterity saving throw, taking 47 fire damage (5d10+22) and is lit on fire on a failed save, or half as much damage and is not lit on fire on a successful save. A creature on fire takes 23 (3d10+6) fire damage at the start of each round until the creature uses an action to douse the flames.
• Wind strike: The Arcane Rubicon sends out a wave of wind in a 40-foot radius sphere centered on itself. Each creature within that sphere must make a DC 18 Strength saving throw, taking 25 (3d8+13) bludgeoning damage being pushed 30 feet back and knocked prone on a failed save, or take half as much damage not move and not be knocked prone on a successful one.
• Flooding: The Arcane Rubicon sends out a wave of flooding in a 30-foot radius around the elemental. The water is 3 feet deep. Any medium creatures in the area must make a DC 18 strength check or be knocked prone and submerged. Any creature on the ground in the area's walking speed is reduced to a quarter, except the elemental. If a creature has a swimming speed, they can use that to move through the area.

Arcane Strike. Melee Weapon Attack: +10 to hit, reach 15 ft., 2 targets within 5 feet of each other. Hit: 15 (1d12 + 9) Force damage. 

Arcane bolt. Ranged Weapon Attack: +10 to hit, range 50/150 ft., 1 target. Hit:20 (4d8 + 6) Force damage.

Spell reflection (recharge 5/6): The Arcane elemental, in reaction to a spell with the Arcane elemental as the target or with the Arcane elemental in the area of effect, the caster being the target or the area of effect centered on the caster.

The Arcane Rubicon has 6 legendary actions. It can use one of the legendary actions options below, but only at the end of its turn. The Arcane Rubicon regains 1 spent action at the end of the round.

• Arcane casting (2 actions): The Arcane Rubicon uses it's Arcane casting action.
• Magic disruption (2 actions): The Arcane Rubicon summons an area of effect in a 15-foot sphere centered on itself. The sphere moves with the Arcane Rubicon and dissipates at the end of the round. For all creatures of the Arcane Rubicon's choosing, the following things happen:
  • No concentration spells can be cast. If the targeted creature(s) has a concentration spell cast when they enter the area of effect, then the spell immediately ends.
  • When a creature casts a spell, the spell has a 50% chance of failing. It has another 1% chance after that of instead becoming a 5-foot radius explosion centered on the caster. Each creature in the area takes 23 (4d8+7) force damage.
  • Any instantaneous spells have a 50% chance of failing when they enter the area.
• Force sweep (4 actions): In a 30-foot sphere around the Arcane Rubicon, an explosion takes effect. Any creatures within that area must make a DC 17 Constitution save or take 78 (9d10+33) force damage and be stunned until the end of their next turn on a failed save, or half as much damage and not be stunned on a successful one. 
• Multiattack (1 action): The Arcane Rubicon makes 2 Arcane strike or Arcane bolt attacks.