The Mireheart

Two entrances lead in from Grimfen: the northern one (23,18) and the southern one (26,23). Right by the northern entrance you'll find a resurrection shrine (27,12). The grate near the southern stretch (26,21) has its chain button only on the south side, so coming in from the southern entrance you can open it as a shortcut into the rest of the cave.

Bogmaws attack among the egg sacs (22,12). Crimson roots creep from the walls and out of the murky water (16,11), and whatever feeds them lies deeper beneath the swamp – it's worth destroying before you push on. That's what the exit to The Rootgrave beside the southern entrance is for (26,24).

Four pink fountains stand in the north (27,4). Drink once from each; any hero can do it and the order doesn't matter. Expect the costs: a random permanent mutation, poison, ooze corrosion, and a freeze with 100 ice damage. After the fourth drink the alcove in the north wall opens and gives you Whispering Armor and 120 XP. Do this with a healthy party, not on your way to the boss.

A hidden button in the wall near the northern entrance (26,18) slides open the wall to the east, revealing an alcove with a Ring of Wisdom (28,18). In the west there's a chest with Armor of Emberspike and the Recipe: Potion of Resurrection (14,30), and beside it an alcove with a Wardstone Elixir, Potion of Mana and Thunderwake Draught (14,31).

A grate blocks the way into the boss area (7,19). Press both skull buttons next to it to open it (7,18). That grate, and the door in the south (10,27), also open once The Thing is dead.

The fight (7,22) comes in two phases. First six Tentacles rise from the pool – fire works on them and every hit they land causes disease, so keep cures handy. Then The Thing itself comes up. Before that you can throw items into its maw and make the fight far shorter: Torren's tincture sets it to 40% HP, Toast stuns it and a Red Mushroom blinds it. If you destroyed all four Tentacle Roots in The Rootgrave beforehand, it goes into the fight weakened.

Afterwards head to the southern part and look for the metal button hidden behind the banner (8,33). It opens the secret wall into a hidden area where the Second Runic Segment sits on a pedestal (9,33), with the Ancient Book: The Mire Warden a few tiles beyond (6,35). Skip this detour and you'll be one segment short for the Gate of Underkeep Tower.