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Build 1: Disruption > Soul Catcher > Soul Catcher > Poison > Soul Catcher > Ult > Soul Catcher
This build used to be a standard of this hero. And it still should be in a lot of cases. It's useful when your team has a lot of front load damage to follow after the disruption.
However, if you take this route and fall behind, it can get quite difficult for SD. Here are some reasons why:
You've put 4/7 skill points into this spell. That's a lot of investment, and you best make sure to use this spell effectively. That's not always an easy task, especially when you are behind. This is because with this build, it assumes that you are the initiator. But when behind, you may not have the luxury to use disruption offensively. And when behind, SD must avoid becoming food to those who can blow him up, so he can't just go in for raw soul catchers. He may have to sit behind and react to the situation with his disruptions and ultimate. But all those times in fights when his soul catcher is not on cooldown is a wasted potential of skill points. He also has very little ways to catch up when behind with this build. It's based on getting kills, and if you can't get kills, what can you really do?
Build 2: Disruption > Soul Catcher > Poison > Poison > Poison > Ult > Poison
This is sort of the lesser variance build of the one above. You have a point into soul catcher early for the aggression, then you transition to a more reliable build. The weakness of thils build is that while you are very strong at lvl1 and 2, you aren't quite so potent at lvl3 and 4. But it sets you up nicely for a mid game strength. This build is nice for 3v1 situations, where you want to maximize lvl1 and 2. Be it killing the suicide, or rotating to mid for a gank.
Poison builds are nice because you have the option of farming woods if you need to. You can stay back and be pushing buttons without sacrificing position and disruption cooldown. It's a more safer and reliable build.
Build 3: Disruption > Poison > Poison > Soul Catcher > Poison > Ult > Poison
Another variant of second build. The nice thing about this build is that you are very potent at lvl3 and beyond. But you sort of suck at lvl2. This build would be nice in tri vs. tri clash. SD is naturally a very good defensive hero that can buy a lot of time in fights, and poison can net him a lot of value there.
Build 4: Disruption builds
Useful if the opposing team has a stat hero like Morphling, Phantom Lancer and Antimage. It's still not a bad idea however, to max poison first then disruption. Assuming you have a decent game, disruption will be potent enough by the time their carries get strong. But if you think you are going to have difficult time acquring exp throughout the game, then better start putting those points into disruption early.
This is already a lot, but it's a shame not more can exist. This is because SD's ult is such a powerful option, you must take it. If that weren't the case, then you might have more builds. Still, this is plenty and the four listed builds offer a very unique purpose for every situation.