What is the rarity of my homebrew Glass Sword?

I was writing up some magical weapons and had a fond remembrance of the "Glass Sword" from Ultima VII (apparently V/VI/IX as well). While I didn’t want to preserve the "save or die" aspect, I wanted to preserve a similar feel of "unusual, fragile, and deadly", and came up with this:

Glass Sword

Weapon (longsword) [no attunement required]

Beautiful and deadly, it is as sharp as it is fragile. This sword deals 2d20 (11) slashing damage. Counts as magic for the purpose of piercing resistance or immunity. On a successful hit (whether or not damage is dealt) roll a d20 to see if the sword shatters – if it does, add an additional 2d20 + 10 (21) damage. The sword initially shatters on a roll of 5 or lower, but this increases by 1 with any successful attack.

Since it will survive for well under 4 hits on average, I believe it is best compared to consumable magical items (and the corresponding scrolls), e.g.:

  • [RARE] Necklace of fireballs – 4-9 instances of 8d6 (28) fire damage, or 0-1 of 14d6 (49)
  • [RARE] 4/5th level spell scroll (~30/40 damage, aoe)
  • [VERY RARE] 6/7/8th level spell scroll (~50/50/40 damage, aoe, various effects)
  • [LEGENDARY] 9th level spell scroll (~110 damage, aoe)

and seems to fall a bit short of Very Rare.