Blood Moon is a three mana Red Enchantment that turns every nonbasic Land into a Mountain
You could also go with a long term plan model: Test of Patience R Sorcery (R) Search your library for a card, shuffle your library, then shuffle that card into the top three cards of your library
The stipulation of only creature spells hurts a bit, but this is definitely a sideboard card to bring in against creature heavy aggro decks, and not versus control decks
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Overlord of the Hauntwoods Three generic, two green enchantment creature Avatar Horror 6/5 Mythic Rare Impending 4 One generic, two green (If you cast this spell for its impending cost, it enters with four time counters on it and isn't a creature until the last is removed
Enchantment Color tag templates(None) Mana Vault This artifact doesn't untap during your untap step