Yes, the activation cost is steeper than that of most creature-lands, but its a worthy price for the chance to bash for 20, 30, or even larger chunks of life
The card is played by having a creature you control climb onto the card (which is probably supposed to mean that you place the creature on top of the card itself), and then "slide them on their merry way"
Elf decks already have Priest of Titania, but other go-wide builds that arent Elf tribal will be interested in this, especially if they dont go tall enough for Selvala, Heart of the Wilds
It may be tempting to stack everything onto your commander and start swinging, but that can make Killian a huge target very quickly
WR Control, by ArsenalMunch (5-0, Modern League) Let's turn to another list by a prominent pro (ArsenalMunch is the MTGO name of Shaun McLaren)