Narvi's Belt is often a key card in Dwarf decks for it's amazing flexibility. You'll almost always have the sphere match with Dain. It's not restricted, and you could slap it on a super Gimli or Gloin even outside of a classic dwarf swarm deck. One common use for me is in a Leadership/Lore dwarf deck. I put it on Bifur, along with Steward of Gondor. Then Erebor Battle Master and Test of Will can slot right in.
I also like to use it to get #Burning Brand on #Dain, or any other non-Lore dwarf hero of your choice — Quetzal513 78
A Test of Will is a response. So to play it with Narvi’s Belt, you’d have to use the Belt in an Action window before the Treachery card is dealt. — Nystrum 30