While it does the usual when this creature dies, return it to the battlefield, it also deals one damage to each opponent, which is a neat little twist
Currently pop 4 with CGC and I know of a few other ungraded
The spell also has the added benefit of replacing itself by drawing you a card when it is cast as long as you control at least one merfolk, letting it still maintain its usefulness over the course of a game
Beyond just being a great card when played straight, Estrid also has combo potential: Estrid, the Masked + The Chain Veil + enchanted lands lets you produce infinite mana by using her -1 to enchant the Veil, then using the Veil to +2 multiple time to untap Veil + lands to go up in net mana
People will be afraid to attack into your vehicle if you have a green mana open
When there are nine or more incarnation counters on this enchantment, exile it