FROM: https://infinite.tcgplayer.com/article/Modern-Golgari-Rock-on-a-Budget/7acade58-202a-4542-bdfb-3949d88aa9e0/

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Mainboard

2 Assassin's Trophy, replacing 1 Maelstrom Pulse and 1 Unearth

2 Nurturing Peatland, replacing 2 Swamp

1 Twilight Mire, replacing 1 Forest

Once you're ready to invest in the high-end cards, I would focus on Tarmogoyf and Liliana of the Veil first. You might think you need to pick up Verdant Catacombs next to improve your mana consistency. However, in a strategy such as Golgari Rock, you'll get more from improving your threats than from optimizing your mana. In Modern, players will often tell you to acquire fetch lands as soon as possible, and while I'm usually one of those players, I think Golgari Rock is the exception to the rule. Verdant Catacombs is unreasonably expensive right now, and likely to be reprinted in Commander Legends, which will lower the price. The addition of Nurturing Peatland and Twilight Mire will significantly offset any fixing issues you could encounter. >>