Aettir and Priwen turns a healthy life total into a creature's base power and toughness. The equipped creature becomes X/X, where X is your life total, allowing a small evasive body to become an enormous threat in a life-gain deck. The size keeps changing as your life total changes rather than being fixed when the Equipment attaches.
Flying, trample, and other evasion help convert that size into damage. Lifelink is particularly appealing because a large hit can raise your life total and increase the creature's base size again. A commander carrying the Equipment can also threaten a major commander-damage hit, provided it can actually connect with a player.
The costs are substantial: six mana to cast and five to equip normally. Stoneforge Mystic can find the Equipment, while Puresteel Paladin or other equip support can help make attachment more practical. Ardenn, Intrepid Archaeologist offers another route to moving it without paying the normal equip cost. Those support cards matter because a powerful Equipment sitting unattached does not affect combat.
At forty life, the Equipment sets the carrier to a base 40/40 before other applicable modifications. At fifteen life, that base becomes 15/15. This makes the card especially sensitive to when damage and life-gain events happen, so check the current life total before committing to a combat calculation.
Setting base power and toughness does not erase counters or other applicable bonuses, so the final creature can be larger still. Losing life can shrink it just as quickly, and removal aimed at the carrier can waste a large mana investment. Use the Equipment when the deck already maintains a strong life total and has dependable carriers and attachment support. That combination makes the dramatic size change a usable attack plan rather than an expensive promise.
