Murasame isn't a standard Magic keyword—it's a specific legendary Equipment card, Murasame, the Demon Blade, originally from the Kamigawa block and reprinted in Modern Horizons 3. It grants the equipped creature a hefty power/toughness boost and lets you sacrifice Murasame to destroy a creature that deals combat damage to its bearer, making it a strong defensive and aggressive tool.
The common confusion is treating it like an evergreen ability rather than a unique card. Its destruction trigger is a one-shot effect tied to Murasame leaving play, not a repeatable shield. You're choosing whether to sacrifice the Equipment in exchange for removal, so timing matters.
In Commander, Murasame shines on a commander you want to protect and pump, since the Equipment sticks around until you opt to cash it in for spot removal during combat.