Cayth does not directly synergise with the energy theme, though this is a good time to note that energy is stored as counters on a player and can be increased every time you proliferate
Its not free, but zero mana is zero mana, and thats worth something I guess
As one of the three leaders introduced in EB-01 Memorial Collection, this Kyros is one of the most beautiful-looking cards in the set
Furthermore, the pull rate in Prismatic Evolutions is 1 in 45 packs, upgraded from the 1 in 90 packs, so you have better odds of pulling for Special Illustration Rares
Generally you only pump the 4th through 7th mana into this when you have nothing better to cast (or bluff) meaning you don't feel to ripped off when they do kill it in response to a level up
But theres a core constituency of Magic who fell hard for the Elder Dragons in Legends, the ripple effect of which has affected the way we all play Magic (and thus the way Wizards designs Magic)