General

This page doesn't deal specifically about .ega files, but with EGA in Eye of the Beholder in a more general matter. And first of all, all the EGA-files are bundled with the PC VGA-version, they exists in the various .PAK files.

The EGA graphics mode

The mode used in Eye of the Beholder is 320x200. Each nibble of each byte of the screen is a 4-bit index color look up in the current EGA-palette used.
EGA uses only 16 colors out of a 64 color palette. The 64 color palette is a simple RRGGBB palette.

Files used in the EGA-version

*.EGA

All except one of these files are CPS-compressed plain images, resulting in a 64000 bytes big files. 320x200 pixels. Each pixel uses only the 4-lower bits which is a index color lookup in the EGA-palette used.
Files BLUE.EGA, BRICK1.EGA, BRICK2.EGA, BRICK3.EGA, DROW.EGA uses CPS-compression format 1.
The file ITEMRMP.EGA hasn't been analysed yet but most probably contains palette mapping per item-image.

*.EMP
The EMP-files are exactly the same as the VMP-files, but instead of indexing blocks into VCN-files, they index blocks into ECN-files.

*.ECN
All ECN-files are CPS-compressed. The ECN-files are the EGA-version of the VCN-files, in the following format:
struct ECN
{
   unsigned short nbrOfBlocks;
   unsigned char zeroes[32];
   struct Block
   {
     unsigned char rawData[4][8];
   } blocks[nbrOfBlocks]
};
The rawData contains the raw pixels. Each byte is 2 pixels. High nibble is the left-most pixel, low nibble is the right-most pixel. Each pixels is an index color lookup in the currently used EGA-palette.

PALETTE.COL
Standard 768-byte palette .col file. Only the first 16 colors are used, and each of those colors only uses the %00110000 bits.