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.

Overall structure:

Offset Size (datatype) Description
0x0000 2 (word)

Effect type

0x0002 1 (byte)

Target type

  • 0=None
  • 1=Self (pre-projectile)
  • 2=Pre-target
  • 3=Party
  • 4=Everyone (inc. party)
  • 5=Everyone (excl. party)
0x0003 1 (byte)


0x0004 4 (dword)

Parameter 1

0x0008 4 (dword)

Parameter 2

0x000c 1 (byte)

Timing Mode

0x000d 1 (byte)


0x000e 4 (dword)


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 0-probability.
Note: Other engines treat probability as two values, effects are applied in the range probability <= x <= probability.
Probability range is 0-99

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