Hmm... I think Leithos isn't the best hero, although I used to pick him before, but now I realize he's just average. His abilities and poisons work well against Stalkers, but you can achieve the same with a Sanitarian while the Stalkers are in invisibility; the Sanitarian casts spells, BUT he'll use more mana!
Also, his ability is quite questionable, as it rarely appears at the beginning, and by the end, it's already weak. The most important thing is that he gets dark magic at the beginning, which is a waste of a skill slot for a Warlock; there are much better skills.
However, it's worth noting that the hero isn't exactly bad; he's extremely questionable, and with the right tactics and in a specific situation, he can be quite decent. Even dark magic can be useful, but with a Sorceress and the Warlock's Chaos spell, it will be much better!