The Sword of Secrets is cut from a single block of jet black obsidian. In game terms it is treated as a power weapon that also allows the wielder to strike as if they had Strength 6 in close combat.
Errata — Chapter Approved: Codex: Dark Angels Q&A (v2.0): Since no '*' appears next to this item in the wargear list, by the letter of the rules a character wearing Terminator armour cannot take it. However, as amended in WD266, the Book of Salvation, all Standards, and the Sword of Secrets can be used by models in Terminator armour.