EFF file format
Applies to:
BG1: TotS, BG2, BG2: ToB, IWD, IWD:HoW, IWD:TotL, PST
General Description
This file format describes an effect (opcode) and its parameters. The format is only ever found embedded in other files. The engine appears to roll a probability for each valid target type, rather than one probability per attack.
Offset  Size (data type)  Description 

0x0000  2 (word)  Effect type 
0x0002  1 (byte)  Target type

0x0003  1 (byte)  Power 
0x0004  4 (dword)  Parameter 1 
0x0008  4 (dword)  Parameter 2 
0x000c  1 (byte)  Timing Mode 
0x000d  1 (byte)  Dispel/Resistance 
0x000e  4 (dword)  Duration 
0x0012  1 (byte)  Probability 1 
0x0013  1 (byte)  Probability 2 Note: The BG1 engine treats probability as a single word, effects are applied in the range 0probability. Note: Other engines treat probability as two values, effects are applied in the range probability <= x <= probability. Probability range is 099 
0x0014  8 (resref)  Resref key 
0x001c  4 (dword)  Dice count 
0x0020  4 (dword)  Dice sides 
0x0024  4 (dword)  Saving throw type 
0x0028  4 (dword)  Saving throw bonus 
0x002c  4 (dword)  Unknown 