Mountain

Frillscare Mentor

When Frillscare Mentor enters the battlefield, put a menace counter on target non-Human creature you control. (It can't be blocked except by two or more creatures.)
{2}{R}, {T}: Put a +1/+1 counter on each creature you control with menace.

Mayhem Patrol

Menace (This creature can't be blocked except by two or more creatures.)
Whenever Mayhem Patrol attacks, target creature gets +1/+0 until end of turn.
Blitz {1}{R} (If you cast this spell for its blitz cost, it gains haste and "When this creature dies, draw a card." Sacrifice it at the beginning of the next end step.)

Loathsome Curator

Exploit (When this creature enters the battlefield, you may sacrifice a creature.)
Menace
When Loathsome Curator exploits a creature, destroy target creature you don't control with mana value 3 or less.

Gluttonous Slug

Menace
Evolve (Whenever a creature enters the battlefield under your control, if that creature has greater power or toughness than this creature, put a +1/+1 counter on this creature.)

Shock

Tentative Connection

This spell costs {3} less to cast if you control a creature with menace.
Gain control of target creature until end of turn. Untap that creature. It gains haste until end of turn.

Burn Bright

Creatures you control get +2/+0 until end of turn.

Tramway Station

Tramway Station enters the battlefield tapped.
{T}: Add {B} or {R}.
{2}{B}{R}, {T}, Sacrifice Tramway Station: Draw a card.

Rakdos Trumpeter

Menace (This creature can't be blocked except by two or more creatures.)
{3}{R}: Rakdos Trumpeter gets +2/+0 until end of turn.

Purphoros's Emissary

Bestow {6}{R} (If you cast this card for its bestow cost, it's an Aura spell with enchant creature. It becomes a creature again if it's not attached to a creature.)
Menace (This creature can't be blocked except by two or more creatures.)
Enchanted creature gets +3/+3 and has menace.

Balduvian Atrocity

Kicker {R} (You may pay an additional {R} as you cast this spell.)
Menace
When Balduvian Atrocity enters the battlefield, if it was kicked, return target creature card with mana value 3 or less from your graveyard to the battlefield. It gains haste. Sacrifice it at the beginning of the next end step.

Flowstone Kavu

Menace (This creature can't be blocked except by two or more creatures.)
{R}: Flowstone Kavu gets +1/-1 until end of turn.

Strangling Soot

Destroy target creature with toughness 3 or less.
Flashback {5}{R} (You may cast this card from your graveyard for its flashback cost. Then exile it.)