Risk of Rain

Open one golden chest.

The Explorer's Key is useable item that is unlocked by default. When used, it instantly unlocks all chests within a certain radius. Golden chests can spawn in most levels, but are more likely to spawn in later ones. Progress until you find an Explorer's Key, then keep it until you find a golden chest. Use the key within range of that chest will earn the achievement, as well as the Captain's Brooch item.

Keycards only appear during the final level. They are used to open optional rooms to collect items and devices used against the final Boss. They can drop from enemies and "golden boxes" (not golden chests). Only four of them appear per run. Play through the final level, collecting everything, but do not use any keycards.

Play level 4 "Hive Cluster" and find the bloated survivor at the bottom right corner of the map.

Lemurians are the small purple enemies that appear in the first level. If you did not earn the achievement through natural progression, start a new game and simply keep playing the first level.