On the front, Cosima is a little hard to parse, but essentially, she works like so: at the beginning of your upkeep, you exile Cosima
Magic: The Gatherings The Brothers War feels like its almost the pinnacle of the trading card game when it comes to a sheer volume of lore
His true power, however, is his ability to create a 0/0 Construct token at the beginning of your end step
(Until your next turn, that creature attacks each combat if able and attacks a player other than you if able.) If the target creature is an illegal target as Red Death's ability tries to resolve, it won't resolve and none of its effects happen
You can't look at them beforehand though, so you've really got to use your finely-honed instincts to find the card you need