Shadow Sigil is one of the five Sigils, Eternal's basic Power cards. You may have for than 4 copies of any Sigil in your decks.
How to Get Edit
Players start with all the Sigils, so they do not need to be collected. Sigils cannot be crafted and do not appear in packs.
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Premium versions of the Sigils were sold in the in-game Store for a limited time as part of the Founder Packages. 
- ↑ Steam News - 1.23 Patch Notes