It's unclear whether this was the most necessary of the Slivers, but it was the one that frustrated the most opposing plans to deal with the problem and even opened up new game plans when the card advantage resulting from saving your creatures could allow Sliver decks to go the control route when backed up by counters and removal spells
Noctis Lucis Caelum You dont need to be rainbow for Kenrith, the Returned King to be good, as even just being WU makes this a great mana sink
But if you want to include her in your 99, then here are a few suggestions: Boros lists with Winota, Joiner of Forces commander
The ETB creates a white and blue Knight with vigilance (the Knight creature token that shows up in the set), and the back is an enchantment that buffs your team
Plus, the engine isnt necessary for its plan of pure aggression
Thats a lot of potential for such a cheap spell