Oko

Anywhere: an attack from behind kills it silently

The Ustanak's scouts: they're born from an organ on its back. Eyeless, they track by ultrasound, which doesn't pass through walls, and they only look ahead. Their tumours glow blue when calm and red when they detect you; then they shriek and the Ustanak comes. On their own they're harmless. Walk up behind one while its light faces away and press the contextual attack: an instant, silent kill. Running doesn't alert them. Shooting one draws the Ustanak unless it dies in one shot (Elephant Killer or sniper rifle).

The maze: first the one on the ladder (the long, clockwise route); then the 2 to the left of the stairs (the short-route one first, and head back to the stairs); then the one on the parallel path, as you turn the corner; and last the one on the exit stairs (follow it and kill it at the top).