The ETB damage keeps early boards under control, and if you cast it normally, youre left with a double strike creature that closes games quickly
First Strike 702.8
According to the lore text at the bottom of the card, the creatures are trapped on a rock (possibly an asteroid.) Someone left behind an encrypted log recording the existence of slivers, either an unlucky explorer or, perhaps, a scientist involved in experimenting with the creatures, which would be a nice callback to the Riptide Project
I adore this card in my deck, and especially because its only five mana for me
Heck, I still see packs from War of the Spark hanging on the end caps at my local Walgreens
There are so many of these types, so its hard to know which is the best (and if we even want these), besides Haunted Dead (which I think is the best)