"Pay 1 life, Sacrifice this land: Search your library for a [basic land type(s)] card, put it onto the battlefield, then shuffle." A fetch land in MTG is a land that lets you sacrifice it to search your library for a land with the specified land type(s) and put that land onto the battlefield
Combat damage is a recurring theme among rogue commanders, so filling your deck with combat damage payoffs, effects that can make your creatures unblockable, and other evasion abilities like flying, skulk, and menace are extra useful
Crystron Citree is not being reprinted in Supreme Darkness, and since it has only been printed once (back in 2016), Speculators and forward-thinking players were quick to buy out this card when those details were revealed on August 6th
With Marneus, this not only generates massive card advantage but also provides strategic utility by copying impactful creatures like [[Thassa's Oracle]] or mana dorks