Whenever one or more creatures a player controls deal combat damage to you, that player takes the initiative. Whenever you take the initiative and at the beginning of your upkeep, venture into Undercity. (If you're in a dungeon, advance to the next room. If you're not, enter Undercity. You can take the initiative even if you already have it.)
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You can't enter this dungeon unless you "venture into Undercity." Secret Entrance — Search your library for a basic land card, reveal it, put it into your hand, then shuffle. (Leads to: Forge, Lost Well) Forge — Put two +1/+1 counters on target creature. (Leads to: Trap!, Arena) Lost Well — Scry 2. (Leads to: Arena, Stash) Trap! — Target player loses 5 life. (Leads to: Archives) Arena — Goad target creature. (Leads to: Archives, Catacombs) Stash — Create a Treasure token. (Leads to: Catacombs) Archives — Draw a card. (Leads to: Throne of the Dead Three) Catacombs — Create a 4/1 black Skeleton creature token with menace. (Leads to: Throne of the Dead Three) Throne of the Dead Three — Reveal the top ten cards of your library. Put a creature card from among them onto the battlefield with three +1/+1 counters on it. It gains hexproof until your next turn. Then shuffle.