No missable achievements (plus 11 unknown)
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"Beat" Half-Life in record time.
Survive the gonarch
Finish Following Portents
Disembark from the church
Finish The Charnel House
Heroically listen to the entirety of the Following Portents commentary
Check-in at the Franklin Motel
Reunite old foes
Find the kidnapped children
Escape the antlion horde... on horseback!
Discover Operation Exodus