Non Neutonian. The Goose is everywhere and nowhere. Cannot be caught, damaged, destroyed, dispelled, or held captive. Non distinct matter form allows goose to freely exit any containment it encounters, including planes of existence.
Insurmountable Chaos. The Chaos Goose is a nth-level spellcaster. Its spellcasting ability is infinite (spell save DC is insurmountable, +3000 to hit with spell attacks). The Chaos Goose has access to the ability to cause the effects of any d100 effect, and it may choose to cause these effects to be permanent if it so desires.
Chaos. Roll a d100 on a chart of many things, or draw a card from the deck of many things. The resulting effect occurs whether or not it is feasible in your universe.
Melee Attack. When a player crit fails: The goose comes out of nowhere, as players hear a honking in the distance, yanks the weapon of choice out of the player's hand, and chucks it 10 ft in a random direction.
Ranged Attack. You Hear a Honking in the Distance: If a mage uses a spell and crit fails on a roll, the spell reverbs off the honking of the goose and hits a random creature on the battlefield or in the vicinity.
Description
The Chaos Goose looks like a normal size small goose. It just always seems to be conveniently in the wrong place at the right time no matter what. Friend. Foe. It knows no bounds and will casually destroy whatever it wants to, whenever it wants to. There is no use trying to capture it. It always fails. Even the Cult of the Chaos Goose can't invoke it whenever they desire, and if they fail poorly enough, it appears only to harm their own ranks and disappear without a trace. Strangely, the Cult view this as a boon even when it directly destroys their carefully laid plans, taking it to mean that their plans were not what the Chaos Goose intended. But What does it intend, except Chaos itself? The world at large may never know.