While the Rakdos-colored Sauron, the Lidless Eye has strong and straightforward effects, Sauron, the Dark Lord is even mightier, and currently has an impressive $30 pre-order price tag
Its been impactful in many formats, to the point that playing 20 more cards in your deck just to get the companion mechanic going seems like a good idea (sorry Commander players, you cant have a 120 card-deck)
Combined with its easy splashability, it was an attractive and popular card to include in decks that were played in environments that couldnt handle it
Put the chosen cards into your graveyard and the rest into your hand
Selvala's last ability looks at the creatures you control as it resolves to determine how much mana of the chosen color to add
At the beginning of your end step, for each token you control of the chosen type that entered this turn, create a token thats a copy of it