What it does
"Rosarius" isn't a standard Magic keyword or ability—it doesn't appear in any official rules text or card. You may be thinking of a card name, a homebrew/custom card, or perhaps confusing it with something from another game (like Warhammer 40,000, where a Rosarius is a protective device).
If you encountered this on a card, it's likely a custom or proxy creation, and its abilities would be spelled out in full on the card itself rather than functioning as a known keyword. There's no established rules meaning to reference.
If you can share the actual card text or tell me which set or context you saw it in, I'm happy to break down how the ability works, common rules pitfalls, and its relevance in Commander.
Iconic cards with Rosarius
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