There was a weapon for the Defiler in the beta named Venom. It looks kind of like a green fireball. The name of the weapon in Arsenal II is 'Glave Wurm (Unused)' and there is another one named 'Glave Wurm (Hero 2)'. The sprite for Venom is the first Glave Wurm.
There are four different explosion types that can be used for splash damage. They are Radial Splash, Line Splash, Unknown (the first one), and Air Splash. For these to hit units around the impact point, the values Splash 1, Splash 2, and Splash 3 must be greater than zero. These values specify the radius of the area of splash damage. Multiply the value by 16 to get the numbers to put in the boxes like you would for Min. Range and Max. Range. Splash 1 is full damage, Splash 2 is half damage, and Splash 3 is 1/4 damage. Radial Splash hits any units in the radius. Line Splash hits only enemy units. The first Unknown hits any units with the same damage as a nuke. Air Splash hits only enemy units, but is not the same as line splash. If a ground unit is hit, ground units around it take damage, but air units don't. If an air unit is hit, air units around it take damage, but ground units don't.
The unknown in the Behavior box makes the graphic for the weapon moving appear on the target. The graphic stays on the spot where it appeared, rotates for a short time, then explodes. The target takes damage from the attack when it appears and keeps losing the same amount at a certain interval until the graphic disappears. The sprite must have frames for moving, it can't just be a graphic for an explosion. Any graphic that can be used for 'Flies to target' or 'Seeks target' can be used for 'Unknown'. When a weapon has 'Unknown' in the behavior box, changing the other values has more of an effect on what the weapon does than when a weapon isn't using 'Unknown'. If missle type is set to normal and explosion type is set to normal, the attack keeps damaging the target. If missle type is set to bouncing and explosion type is set to one that does splash damage, the attack will be like Psionic Storm. If missle type is set to homing and explosion type is set to one that does splash damage, the attack might be like Irradiate.
Here are the weapons that use some of the above settings.
first Unknown in explosion type box
Unknown in behavior box