Demons are awkward to build and rely heavily on getting specific units to make the lineup work.
Units: Anti-Mage, Terrorblade, Chaos Knight, Shadow Fiend, Doom, Pudge, Necro, Kunkka
Synergies: (2) Demon Hunters, (4) Demons, (3) Warriors, (2) Warlocks, (2) Heartless
The above lineup is a decent example because it benefits from a few secondary synergies that make the overall strat better. The (3) Warriors frontline buys a bit of time, the (2) Heartless increases your DPS further, and the (2) Warlocks are very useful with your high-damage Demons and Demon Hunters.