The forces of the Devil are one of the three prime evils that plans to destroy God's Throne. The Devil player has three heroes in his/her disposal: the Devil, Mammon, and Beelzebub. These heroes are also supported by powerful elite units that assist them in pushing the throne.
The forces of Satan rely primarily on melee units to advance (Mammon with his fellow Behemoths), designed to be used in conjunction with Beelzebub's Dark Swarm, which makes these units impervious to ranged damage (which makes the majority of Heaven's forces ineffective). Because Heaven's forces have an extremely high damage output, dark swarm is almost always required when Mammon with his behemoths or harpies are attacking Heaven's forces.
The player should always be careful to watch for the conditions of their eggs as either a sneaky Zadkiel can usually kill these eggs in 3-4 swipes or the time of the spell's effect may run out, in which case the attacking units will become very vulnerable to the ranged damage of Heaven's units.
Many Messiah players will attempt to use their predators to trap Mammon and his behemoths, so it is important to keep a vigilant eye for the shimmering that indicates the presence of these units.
While Devil may appear to be one of the challenging and daunting roles to master, it can be the simple and the most rewarding. How? The key to playing Devil efficiently and effectly is to be simple.
As a Heaven Besieged player, you must understand that Beelzebubb and Dark Swarm is a game changer and can easily swing the tide of battle to one side or another if used properly or uneffectively. Dark swarm can be used defensively to stave off a flank or a counterattack as it serves as a buffer preventing Heaven's units from entering as their ranged attacks will become almost useless and it would be unwise to enter the spell's AoE as the cloud signals that Mammon and Behemonths will be nearby. It can also be used offensively to force Heaven's units to move positions and respond to what the Devil is doing. When using eggs, use them where you intend to push with Mammon and the Behemonths, Harpies, or Tormented Souls. The purpose of Dark Swarm is to protect your units. Use it where they need to be protected. Plague is just as important as it can whittle down at the hitpoints of Heaven's forces, but it will never lead to a direct kill. Use it against clumped, high hitpoint units that preferably do not have shields. Uriel's goliaths, Jeho's Seraphs, and Mike's bioball are prime targets. The ability is to be used more sparingly, however, as it consumes more energy and doesn't have as much immediate benefits as Dark Swarm.
The Devil is far from being one of Hell's most important heroes. The Devil's strengths are are his supreme movespeed, high attack damage, and, not so importantly, high amount of hitpoints . The Devil, however, has a low range and he's a priority hero to focus fire, as with all of Hell's heroes, making him relatively fragile if not carefully watched or used in the wrong situations. With all this in mind, use the Devil as a tool to snipe weak heroes such as Metatron or any hero that is caught alone. The Devil can also be used to chase down pretty much everything Heaven can offer and can be used to chase away Uriel's Reaper Squad if they're harassing Asmodeus and venturing far into Hell's territory, but chasing after a lone Nathaniel or Ralphael would be another situation to capitalize on the Devil's mobility. Sniping siege tanks at fortified positions is also another responsibility of the Devil.
The Devil's elites are really straightforward in their purpose. Behemoths should always be used with Mammon under Dark Swarm and their splash damage is great for clearing swaths of the weaker angels. Harpies, on the other hand, have high, single target damage that is especially useful for sniping heroes or Heaven's tougher elites such as Chariots. Their main strength comes from their ability to scale high ground and quickly destroy anything they target. They are commonly used against positions fortified by siege tanks. They are more fragile compared to the Behemoths though and need Dark Swarm just as much if not more. Tormented Souls are relatively used at the very beginning to quickly take down Heaven's Gate through a quick rush, throughtout the game as Baneling mines if Heaven is harassing, especially Uriel's Reaper Squad for that matter, or at the very end for a Throne Rush. They are most commonly used under a Dark Swarm and their advance usually leads to Heaven's troops from retreating or facing heavy casualties from the high splash damage that can instantly decimate clumped heroes and elites alike.
With these points in mind, use the Devil's abilities and units for specific situations as you need them.
In the beginning portion of the game, Red's primary objective is to destroy Heaven's Gate to deny Heaven the reinforcements of powerful elite units. Many proficient Messiah and Zadkiel players would be able to halt your advance with cloaked units, so it is necessary to bring Grigori (overseers) to detect them. It is important not to lose them, however, as they are extremely valuable.
Although the gate is the primary goal, the player can also attempt to take down the Enclave through Mammon rushing and the Benediction with harpies.