Is this homebrew entropic spear balanced?

I have created a magical entropic spear that I plan to give to the party when they reach roughly 10th level. Is this balanced?

Entropic Spear

Weapon (spear), rare

This is a +2 magical weapon. On a critical hit, this spear does an extra 2d6 damage of a random type. Roll a d10 on the table below to determine the type:

  1. Acid
  2. Cold
  3. Fire
  4. Force
  5. Lightning
  6. Necrotic
  7. Poison
  8. Psychic
  9. Radiant
  10. Thunder

This spear has 5 charges, regaining 1d4+1 charges at dawn. As a reaction, when an attack hits you, you can expend 1 or more charges to reduce the damage by 1d6 per charge spent. You can only reduce damage of a type that appears on the above table. If you use the last charge, roll a d20 after applying the effects of the item. On a 1, the spear crumbles into dust and cannot be repaired.