Time bomb is a fire damage, sonic damage, knockdown ability in the mechanic tree.
However, it seems to be completely blcoked by spell resistance on most spell resistant mobs, regardless of your characters spell penetration. For example: my caster can clearly break drow spell penetration here in elite heroic Von3 with 4 favored soul levels, and disable evasion by helpless stunning the archers here, but the exploded time bomb fails to register. The only explanation is spell resistance is blocking this ability, although no spell resistance symbols appear. Strangely, even though my character has the proper spell pen feats/past lives/enhancements/gear, and the time bomb should spell pen like soundburst does, it doesn't. Time bomb does not seem to work on drow, duergar, yugoloth, and although I haven't tested it I'm guessing most spell resistant mobs.
It isn't the archers superior balance/dex/reflex, since soundburst helpless stun drops dex bonus to 0, and time bomb doesn't work on caster drow either:
However, it will work on some golems, most notably flesh and iron golems, (but not very often on clay golems?):
In addition, time bomb will work on some smooth slopes, against some mobs:
However, it won't work if there is a small lip, or on all slopes, or if a rock protrudes from the ground in the path. As per Tlorrd and testing, the time bomb generally has to be higher than the mobs to affect them:
The lip/rock appears to entirely stop and absorb time bomb.
Presumably, time bomb is setting off a ground level pulse in 360 degrees, with no vertical component. This is verified by it not hitting flying mobs, like mephits. It can hit some floating mobs, like reapers, that you soundburst stun.
Time bomb doesn't scale off spell power, but does scale on spell crit. I have yet to test its scaling based on sonic/fire caster levels. I also need to see if there is some way to jump cast it, but I don't think so.