Coldiron Blade

Freezing Burst: When the coldiron blade drops to zero hit points or fewer, it explodes in a blast of freezing metal.  Creatures within 10 ft. of it must succeed on a DC 15 Dexterity saving throw, taking 13 (4d6) cold damage or half as much on a successful save.  

Mage Killer: The coldiron blade has resistance to all damage from spells.  In addition, it adds its proficiency bonus to all saving throws from magical sources, and imposes a disadvantage equal to the proficiency bonus on all spell attacks targeting the coldiron blade.

Marked for Death: The coldiron blade has a marked target.  It has advantage on all checks to find that target, as well as to determine the target's motives.  If the coldiron reduces its target's hitpoints to zero or fewer, it may take the Fade into Shadows action without disadvantage on its Stealth check.

Shapechanger: The coldiron blade can assume the form of any Medium humanoid it has seen before.  While in this form, it retains all of its actions and statistics.  As a bonus action, the coldiron may also shape one of its arms into any melee weapon it has previously studied.  Its melee attack damage changes to reflect the current shape.

Crippling Strike: The coldiron blade makes an attack against a target.  If the attack succeeds, the target's speed is halved until it takes a short rest.  May only be used once per encounter.

Multiattack: The coldiron blade makes two melee attacks.

Tainted Mana: The coldiron makes an attack against a target.  If the attack succeeds, the target must succeed on a DC 19 Constitution saving throw or suffer from mana poisoning.  Once the target succeeds, it automatically succeeds on all further saving throws.

Mana Poisoning: Every time a creature with mana poisoning casts a spell, it takes 1d6 poison damage per level of spell slot expended, with a minimum of 1d6 poison damage.  It must succeed on a DC 18 Constitution saving throw or be the target of a greater restoration spell to end the condition.

Melee Attack: Melee Weapon Attack: +6 to hit, reach 5 (10) ft., 1 target. Hit: variable damage, depending on arm form, plus 1d6 cold damage and 1d4 poison damage. 

Sniper's Shot: Ranged Weapon Attack: +7 to hit, range 150/400 ft., 1 target.  Hit: 13 (3d6 +4) piercing damage plus 1d6 cold damage and 1d6 poison damage.

The coldiron blade has 3 legendary actions.  Only one legendary action can be used at a time, and only after another creatures turn.  The coldiron blade regains all spent legendary actions at the beginning of its turn.

Track: If the coldiron blade's target is concealed or in cover, the coldiron makes a Perception check to locate the target and remove all penalties on attacks.  

Freezing Fog: The coldiron blade casts [Tooltip Not Found] centered on itself.  Any creature entering the cloud or starting its turn inside the cloud takes 1d6 cold damage and has it's movement speed reduced by 10 for one round.

Fade into Shadows (Costs 2 Actions): The coldiron blade takes the disengage action and moves up to half its speed.  If at the end of its movement it is in dim light or near cover, it may make a stealth check with disadvantage.