Adamanthril

Adamanthril is indestructible, even more than adamantine. It is cool to the touch, black and very reflective. If something magical is near, it will start to glow purple, which is why magical items made of adamanthril usually glow in some way. It is very smooth usually and light. The surface of adamanthril seems to be constantly moving. Any item made of it is legendary and has at least 10x times the value of a normal legendary item.

It is created with a special forge, where it heats and cools ionized mithril and Adamant. That creates an incredibly powerful alloy. No attempt, be it magical or nonmagical, can deform adamanthril, forcing smiths to have to make an item with the metal before it solidifies for the last time. After it has cooled down completely, nothing can heat it or cool it further. 

Adamanthril Weapons:

If unmagical, the weapon ignores resistances and immunities of nonmagical slashing, piercing and bludgeoning damage.
If magical, the weapon ignores resistances and immunities of magical slashing, piercing and bludgeoning damage.

Every weapon has the light and finesse trait. A weapon with the two-handed trait loses it and instead gets versatile, dealing one dice lower damage if wielded with one hand.

Adamanthril Armor:

Unmagical full plate and chain-mail armor grant you immunity to nonmagical slashing, piercing and bludgeoning damage and resistance to magical slashing, piercing and bludgeoning damage and resistance to fire, cold and acid damage. 
Magical full plate and chain-mail armor grant you immunity to nonmagical slashing, piercing and bludgeoning damage, immunity to magical slashing, piercing and bludgeoning damage and immunity to fire, cold and acid damage. 

Every armor is as heavy as light armor, and you gain no disadvantage on stealth checks and require no strength score to wear it.

Destroying.

There are only two way to destroy Adamanthril.
The first is to have a sphere of annihilation touch it, and the other is by colliding two adamanthril objects together and succeeding a DC 40 strength save.

When adamanthril gets damaged like this, the whole item starts violently unraveling, dealing 100d10+50 force damage per 0.01 pound of adamanthril. the range of the explosion is 400 ft. per 0.01 pound of adamanthril.