Second Sight is 20/20 (or more like 10/10 would use again) Slowking has always been about messing with peoples decks, trying to lock them, trying to control the game state
Zegana also pairs really well with high-powered green cards like Ghalta, Primal Hunger, which can enter the field before Zegana and give you 12 cards in hand and 12 +1/+1 counters
A 4-mana 4/3 creature with a decent keyword is an okay play, though not a great one
Mix in some recursion through cards like Splendid Reclamation and Crucible of Worlds, and you can easily ramp up your mana to do it all again
It triggers when Rattleclaw Mystic is turned face up
Widespread Brutality Prior to LTR, there werent really amass decks, but the newer support makes Widespread Brutality better