With Tergrid's effects, it fits into any forced sacrifice or discard decks, as it's a way to get around the downside of often being one-for-one trades by putting them on your side of the battlefield or having a static ability to force the sacrifice and discard
It started with Bloomburrow and Duskmourn and has led me to the new Final Fantasy precons
Since assassins predominantly have abilities that kill creatures, Mari helps you execute that strategy while providing card draw and tempo advantage
Super Special #4 Command the Dreadhorde In Commander only, where you have 40 starting life
Bad dual lands lead to monocolor decks dominating, and good dual lands allow for tricolored decks to be good which usually lends itself to more enjoyable formats
It will go for hundreds of dollars, potentially thousands if the card ends up being good, and its the one thing you really want to open in the set