You can do this at any point after the creature has been declared as an attacker so anytime from the declare attackers step through the end of your turn
The Great Sleep Necron Dynasties is good deck to buy if you're starting out in the format or if you're a more enfranchised player looking to have a precon to keep together
As it is now, they are just too slow to really be of value to most decks, and youre probably better off playing the depletion lands, like Saprazzan Skerry, instead
Most famous is the Black Lotus, considered to be the most valuable MTG card in existence - an Alpha copy of Black Lotus autographed by artist Christopher Rush sold in 2021 for over $500,000, but even standard copies can fetch tens of thousands at auction
If an effect asks you to choose a creature type, you can't choose outlaw
Other cards have activated abilities that require energy to activate