What it does
"A Thousand Souls Die Every Day" isn't a real Magic keyword or ability—it's not a mechanic that appears on cards or in the comprehensive rules. You won't find it referenced in any official rulebook, set, or card text, so there's nothing functional to explain in terms of gameplay.
If you encountered this phrase, it's likely flavor text, a custom/fan-made card, a deck name, or a misremembered effect. Plenty of cards deal with creatures dying en masse (think Grave Pact, Dictate of Erebos, or board wipes), and "souls dying" is common Orzhov/black flavor language, but none of that constitutes a defined keyword.
If you can share the actual card name or where you saw this, I can identify the real ability it's tied to and walk you through how it works in Commander.
Iconic cards with A Thousand Souls Die Every Day
See all 1 →