The expiry routine of timer 2 (count 5), started when Bilbo enters the place of black spiders (C6A1). The count drops to 4 at the end of the turn he enters, so it reaches 0 at the end of his fourth turn after that: if he is still in room 26, 'the spider web is slowly smothering you', and he dies. In the emulator, entering and waiting three times, then leaving, is safe; waiting a fourth time is fatal.