This achievement is obtained by beating the final boss in Volume 1 of the Bard's Tale Trilogy, found in Mangar's Tower. After completion, you can return to the guild in order to move on to Volume 2 with your characters, carrying over all inventory, character levels and money (you will also have the option to adjust your party's levels and stats to match a more suitable level for the game, or to do a complete reset with everyone at level 1). Of course, to make your life easier, I would transfer just to avoid needing to grind for levels, money and items as much. It will also help you get to Level 50 without such a grind.
Should you need guidance throughout any point in the game, I used this guide (https://sorcerers.net/Games/Bards-Tale-Trilogy/Walkthro....php
) and these maps (https://bardstaleonline.com/bt1/maps?sosi
) to help me along.
The final boss is not easy... you'll need to be level 35+ after some decent HP rolls, and and be prepared to deal with Stone, Possession, and resurrection. Overall I would say completion of Volume 1 takes about 30 hours, including level grinding, farming for weapons and armour, and returning to the shop for selling excess inventory.