These are "secret rare" cards
These creatures all Proliferate or pseudo- Proliferate when they hit a player, making both my commander and the rest of my board slightly scarier each turn
River Song has a surprising number of synergies, with cards like Junktroller and Timestream Navigator benefiting from the deck manipulation while you can force yours opponents to do all the things River Song punishes them for with cards like Field of Ruin and Cleansing Wildfire
"Boy, 6, robbed at knifepoint for Pokmon cards"
There are a myriad of mill effects in black and blue to make sure that this Horror comes out huge and after its on the table things go from bad to worse
It's not a strategy for the faint of heart, and because it requires so much resource investment, it's usually not a good plan to split the difference with another mechanic