+damage work well with ultimate because slow helps landing hits. Anyway it's more important in many many case to aim well soul catcher than having amazing +dmg.
I noticed that manta was really strong on him coupled with a ghinsoo or an orchid (but paper hp)
Linkens is good due to the ganking nature of the hero + it synergize with manta images a bit.
I use to make PT/Orchid(if amazing early) else linkens/Manta and it's over, I don't think have lost a single pub with this hero. As orb I don't really know wich is the best, but satanic, diffusal and mjollnir seem to suck. Stygian is pretty but gives 0 survivability and only damage so imo scadi seems to be the only choice
. With his low str I suggest building some +stats items like linkens and use manta on purged heroes, scadi or ghinsoo as final luxury.
Necronomicon ofc is welcomed if the true sight is needed + the summons will dps hard on a purged and soul catched unit (x2 nuke + mana burn + hits) Never tried but imo it can be core hence this hero can disable a bit and need HPs.
Dagger as 3 uses:
1) try to escape safely with the help of disrupt
2) come easily to contact after a long range purge
3) land low range soul catchers
Imo Dagger should be wanted if you are playing agressively and have already items to kill properly (examples: manta(+linkens), orchid(+ manta), dagon, necro)
Eredar is not a "farmer": 12s to kill a full creep wave assuming you hit everything and you don't waste 0,5/1s at each cast(4stack) is longer than a lot of heroes in the game. Io for example owns creep waves faster with less chances of missing last hits. In the other hand he is an average+ pusher with good last hiting potential.