Soul of New Phyrexia

The Soul cycle was built on a single repeated promise: a six-mana beater that does its best work after it dies, with a graveyard-activated reprise of the same effect for anyone who can pay again. This one's reprise is mass indestructibility, which makes it the most quietly insurance-heavy of the five. The body wins a damage race on its own, but the activated ability is the reason it earns a slot: at any point you can buy your whole board a turn of immunity, blanking a sweeper or letting an alpha strike survive a blocking step intact. Killing it does not turn the ability off; it just moves the same protection one zone over, where the graveyard line lets you reach back for one more shield before exiling it for good. That doubling is the cycle's structural cleverness: the card asks to be answered twice, and the steep activation cost is what keeps a repeatable "your stuff cannot die" effect from being oppressive. Indestructible does not stop exile, bounce, or sacrifice, so the protection is a window rather than a wall, which is exactly the calibration a six-mana artifact creature can support without warping a game.