This achievement is specific to Bard's Tale I: Tales of the Unknown.
To defeat the Crystal Golem, you first need the Crystal Sword, which is obtainable from Harkyn's Castle at N0 E19, so you'll have to wipe out the guardian statues to get in there.
Once inside, head straight east, north into the big room, then take the north eastern door past the golem. Follow it east and straight south past a series of traps and darkness, and you'll run smack into the Jabberwock, which can be a very challenging fight, even with a group appropriate for Harkyn's, so be careful. Down and around from the Jabberwock is the Crystal Sword, which is quite powerful for the level.
At the end of Harkyn's Castle, you will be teleported out... right next to Kylearan's Amber Tower, which is where the Crystal Golem lives. Note that if you leave through a gate, you'll likely have to go back through Harkyn's to get to Kylearan's again.
In Kylearan's, the crystal golem itself is located at N1 E4, which sounds deceptively close to the entrance. To get to it, you'll actually need to go up and around, teleport to the north east corner, go through the darkness to teleport into the center, have the mouth at N10 E13 teleport you (STONE GOLEM is the answer he wants), then go straight down through the darkness on the west side. Go up and down and around, and then follow through the darkness maze (where you'll want the Silver Triangle at the north if you're playing through, but isn't necessary for this) and down to the south.
The crystal golem is down there and must be killed with the crystal sword giving the final death blow.
I take no credit for these, but they're awfully helpful (especially in the darkness maze):https://bardstaleonline.com/bt1/maps/harkyn_1https://bardstaleonline.com/bt1/maps/kylearan_1
And as noted by tony coconuts (for those of you who actually want to continue onward):
...the only way of getting the door at the end of the darkness maze (N6 E1) is to first visit the mouth at N2 E12, it’s in the big darkness room en route to the maze, and answer sinister.