Barathrum the Spiritbreaker a quick guide

overview

Barathrum is, at heart, a 1v1 hero. He can gank fantastically and has the only bash in dota that has the potential to be a literal perma-bash. He can farm well early on with his high damage from Empowering Haste. Unfortunately, he does not have much of a presence in team fights.

The choice between Blademail and BKB is one of survival. If you can live though ulting in with Blademail, use it for the additional damage. Otherwise, get a BKB.

Some details:
Charge gives vision (even on invisible heroes). You can use it to help ganks even if you don't plan to go there.
You can stop the Charge of Darkness powering up by pressing the hotkey again or issuing another command. With maxed Charge and Haste, you only need ~2s to reach max MS so only charge fully if you want the magic immunity.
Bara's bash has the same percentage chance at every level, which is why one level is fine early on.
Bara's ult has a 1 second delay (but you're magic immune). This means it's a bad idea to ult someone that started a TP a second ago (you might end up in their fountain) and a good idea to ult someone you think is about to blink (you will hopefully follow).