Is Sothe worth building?

He's basically the best normally summonable dagger right now so I'd say so. He comes with an amazing base kit and can kill/buff/debuff all at the same time. He's also an amazing team player if you build around him.

He's one of the better offensive daggers and a great dagger unit in general. If he's got some merges then by all means keep on using him

In my opinion, Sothe is the best dagger in the game thanks to his weapon. Especially in rival domains and grand conquests, Sis an unstoppable beast who buffs most of your allies and debuffs most of your enemies. One copy of him should fullfill the role pretty well, but you can merge him if you want (I don't recommend because he he can inherit to others life and death). Build his budget build and give him vantage, as he is strong enough one shot almost any ranged hero he goes against after the buff/debuff from his weapon.

Sothe is amazing. He can buff your team to am effective+13 atk/SPD. He's got the best dagger attack, great speed, innate L&D and to add a cherry to the top spur atk/SPD. Really, put some desperation in him and you'll see some serious endgame.
PS: I know it's very tempting to use as a L&D fodder, but he's way better than that. Trust me.