In that battleground of a ring, the rules aren't nearly so black and white.

Underdome

{T}: Add {C}.
{T}: Add one mana of any color. Spend this mana only to pay Un-costs.

Unsanctioned

{T}: Add {C}.
{T}: Add one mana of any color. Spend this mana only to pay silver-bordered costs.

The mana produced by Underdome’s last ability can be spent to pay any cost imposed by a silver-bordered card. This includes casting a silver-bordered spell, activating the ability of a silver-bordered card, or paying for abilities like the one Infernal Spawn of Infernal Spawn of Evil has.