FMG files contain all strings in the game, localized per language. They can be edited using Yabber, including modifying existing entries and adding new ones. Sometimes the ids in the file are arbitrary, and sometimes they have to match the ids of params (e.g. item ids), so they are a useful way to find out which param rows correspond to which items.

See Modding Tools Database for game unpacking tools. It is necessary to unpack the full game to view/edit FMG files. See also the Glossary for how the game classifies different item types.

Below is a quick overview for which files contain what. If you can't find what you need here, your best chance is probably unpacking all of them and searching inside.

Dark Souls I

In DS1, all messages are in subdirectories of msg, like msg\ENGLISH. All item names and descriptions are in item.msgbnd.dcx, plus things like spell names, area title names, and NPC/enemy names. Most everything else is in menu.msgbnd.dcx, which can be unpacked with Yabber.

In PTDE, FMG file names are in Japanese:

In Remastered, most names are in English and fairly self-explanatory. The main non-obvious files are

Dark Souls II

In DS2, all messages are in subdirectories of menu\text, like menu\text\english. Dialogue is in the talk subdirectory, like menu\text\english\talk, per map. These are also self-explanatory.

Names above boss health bars are found in menu\text\english\talk\charaname.fmg.

Dark Souls III and Sekiro

In later games, all messages are in subdirectories of msg, like msg\engus. Like DS1 they are contained in bnds, with FMG names are in Japanese. In DS3 current patch, these are in item_dlc2.msgbnd.dcx and menu_dlc2.msgbnd.dcx, even if you don't have DLC installed. The FMG names themselves have _dlc1 and _dlc2 variants, for items/NPCs/etc. in the DLCs. The notable names are as follows: