Long story short, it's a pain to master set, but an absolute joy to rip open
With this future card, you can attach up to six, assuming you have a bench full of future mons
Other Elves, like Elvish Archdruid, Circle of Dreams Druid, and Priest of Titania, will produce more mana as your Elf army grows, while Knotvine Mystic, Druid of the Anima, Bloom Tender, and Quirion Elves will all fix your mana by providing green, red, or white
Evasion is something that merfolk do really well, so combat damage triggers are another great payoff for merfolk decks
It looks like Sivitri was the one Legends legend to receive an upgrade from creature to Planeswalker