Shadow Crystal is a mysterious key item obtained from defeating a bonus boss in Deltarune (Jevil in Chapter 1 and Spamton NEO in Chapter 2). They can be given to Seam, who explains that the crystals may be found with powerful enemies, and that said crystals are invisible but still cast a shadow.
Upon collecting the second one from Spamton and bringing it to Seam, they explain that the next foe that carries one of these may be completely unbeatable without an item known as the Shadow Mantle, which they intend to give to the party, but cannot find.
If the party has defeated Jevil in any save file (but did not in the current save file, or otherwise lost Jevil's reward item), they can find the first Shadow Crystal (and the Devilsknife or Jevilstail) by inspecting the hole in the cliff to the left of Castle Town. If the party gives the second Shadow Crystal to Seam but have not given them the first, Seam hints at this, mentioning that the party did not collect the crystal from Jevil but that if they ever have defeated him then it will show up nearby.
- Using the Shadow Crystal makes Kris see something related to the theme of that chapter's Dark World. This effect quickly passes and is not repeatable.
- In Chapter 1, Kris sees "toys strewn on the floor."
- In Chapter 2, Kris sees the computer lab from the Library.
- Kris cannot see anything in Castle Town, even on first use.
The Glass is the Light World form of the Shadow Crystal, separate from the Ball of Junk. Using it causes Kris to look through it. It cannot be dropped, as "the thought of discarding it felt very wrong." Using the Glass makes Kris see through it, either seeing Susie glaring at them if she is following, otherwise seeing through their hand.
- Prior to version v1.06 of Deltarune Chapter 2, the Glass did not appear in the Light World inventory due to a glitch.
- There is an unused item called the Pure Crystal, whose description suggests it may have been a Shadow Crystal purified by Seam.
- DELTARUNE Chapter 1&2 has been updated to version 1.06 to make a few minor adjustments and bugfixes - @UnderTale on Twitter, September 23, 2021.