Unstable Obelisk

{T}: Add {C}.
{7}, {T}, Sacrifice Unstable Obelisk: Destroy target permanent.

Fire Diamond

Fire Diamond enters tapped.
{T}: Add {R}.

Arena of the Ancients

Legendary creatures don't untap during their controllers' untap steps.
When Arena of the Ancients enters, tap all legendary creatures.

Sol Ring

Mountain

Gilded Lotus

Pia Nalaar

When Pia Nalaar enters, create a 1/1 colorless Thopter artifact creature token with flying.
{1}{R}: Target artifact creature gets +1/+0 until end of turn.
{1}, Sacrifice an artifact: Target creature can't block this turn.