Basically it's a different version of the warlock spell Corruption, it does a certain amount of damage over time. What's different about this spell is that it has its own, built in finishing move. When the target has sustained 1044 total damage, whether it's from Seed of Corruption, other spells you cast, or spells other players do, the seed "goes off" and inflicts all hostile targets within 15 yards with a one-time shadow damage spell. This spell will kind of chain react with other seeds, so if you cast it on, say, 4 targets, the first one exploding will contribute to the other targets exploding. Finally, I can't throw this and Corruption on the same target. Which is sort of a bummer, really.
This spell is incredibly useful in a few different situations. The spell does, at a minimum, about the same damage/mana as Corruption on a single target. When there are multiple close-packed targets, the efficiency really starts to shine. For the downsides, read on.
The biggest downside is that the spell will hardly ever last the full 18 seconds. If you have a reasonable amount of shadow damage gear, then the target will take 3, maybe 4 ticks of the damage-over-time portion of the spell before it explodes. Second, this spell does not trigger the Nightfall talent, so it represents a small downtick in DPS.
Finally there's the whole concept of this thing being essentially a timebomb area of effect spell. Let's say it's the only thing ticking on that runner that just stunned your tank in a crowded instance. The runner gets 3-4 yards away and goes kablooie, right next to his 5 good friends. Well, the mob dies, but his friends are ticked, and they're ticked at the warlock that cast it, not at the warrior that needs to tank them.
I'd heartily recommend against using this inside of buildings when mobs are in adjoining rooms. Some walls aren't as solid as others (I'm looking at you Durnholde Keep). This may result in you doing damage to mobs unintentionally, which will make the mobs very very angry indeed.
In PvP, dispelling this spell will save your group. If you don't have someone dispelling you, you have two options. If you're alliance and in my battlegroup, run as far away from me as possible. If you're horde and in my battlegroup, you should show your support to your priests, shamans, druids, and now paladins for their selfless dedication to the healing arts and go give them a nice bear hug.
Action shots? Sure, I got action shots. Many Firewing Defenders died to bring us this information. First, the "tick tick" shot. The hits for 98 are my voidwalker hitting in his typical "Girl Scout in Plate Mail" mode, the hit for 407 is a tick of Seed of Corruption:
This is what it looks like when Seed of Corruption goes off:
For all the warlock haters out there, there's just one spell left, and it's a group buff spell.