Rest in Peace stops Malevolent Rumble/Green Sun's Zenith grindy recursion and denies their use of Shifting Woodland and Boseiju's land-rebuy lines, while also hosing any escape-style plans. Obsidian Charmaw comes in as a cheap flying beater that also blows up a nonbasic land like Urza's Saga, The Mycosynth Gardens, or a Gruul Turf/Simic Growth Chamber bounce-land, disrupting their mana base significantly. Spell Pierce and Thraben Charm are the weakest cards against a deck that doesn't attack with many relevant creatures or play many enchantments worth hitting.