The Lost Soul in Doom 3 looks vastly different from it's classic counterpart. It is a flying demon consisting of a human head mounted on a flying rocket engine contraption with jaws torn apart to allow it to open its mouth wider than a normal human could and its teeth have mutated to long, pointy, needle-like teeth. When alerted, they make a high-pitched shriek, accompanied by the sound of the rocket engine that powers them.
The first appearance of a Lost Soul occurs in a grotesque cutscene with Theresa Chasar, one of the few female UAC employees. When the marine approaches, she whispers "help me", then her facial skin starts to shrivel inwards while acquiring a horrible pale color, and a Lost Soul bursts out of her head, ripping out her spinal cord in the process. It could be possible that a Lost Soul needs to manifest itself within a body; this is supported by the cutscene and the rest of the Lost Souls could have come from the bodies with the damaged faces as well as the ones without heads.
They hover through the air at average speed and attack by abruptly rocketing in a straight line towards the player at high speed and ramming into them, similar to the classic Lost Souls. Unlike the classic Lost Souls, Doom 3 Lost Souls can cover a larger area and can change course if necessary. The Lost Soul is usually found in packs with other Lost Souls (which was often the case in the classic Doom games as well).
While the Doom 3's version of the Lost Soul is weaker compared to its classic version (a single punch or 2-4 pistol bullets can take them down instantly), it is a much more difficult target to fight against. Their smaller size and the lack of space for the player to dodge their charge attacks can make a Lost Soul potentially more dangerous than the classic version. If they survive the first shot, it is more than enough time for a Lost Soul to retaliate against the player.
The general strategy is to maintain a reasonable distance away from these enemies, especially when coming up against them in swarms. The Shotgun is ideal against fighting an individual Lost Soul at point blank range. However, it's low rate of fire and reduced effectiveness at long range makes this weapon unviable when fighting a horde of them. The Machine Gun or the Plasma Gun are more efficient for taking down a horde of them, thanks to their high accuracy and rapid rate of fire. A skilled player can also rely on the Pistol, only if they are in a safe distance from a horde of Lost Souls. But if they close in, switch to either the Machine Gun or Plasma Gun.