Black Hand is not a title, it is a trait unrelated to any sect. The trait is not lost if the vampire changes sect.
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Al-Muntathir, God's Witness
G5Sabbat: If you control 4 or more True Brujah, burn Al-Muntathir. He cannot use cards requiring Celerity . Black Hand. Scarce.
Rulings
Can use a multi-discipline card with a potential (unused) Celerity effect.
The scarce penalty considers the apparent clan while controlled (in case of Clan Impersonation and Banishment).