Single-way combat

Single-way combat area, also known as a Single-combat area, is an area of the RuneScape world where a player or monster can be attacked by only one player or monster at a time. Most areas of the world are single-way combat. Those that are not are multi-combat areas, where a player or monster can be attacked by several players or monsters at a time. Multi-combat areas are identified by an icon of two crossed swords which appears in the top right-hand corner of the interface or under the minimap. If this icon is not present, then the area is single-way combat.

Exceptions
These monsters do not follow single-way combat rules:

Barrows brothers
If the player is under attack by another monster in the crypt and a brother spawns, the brother will override their current attack, causing the player to end combat with the monster and start with said brother. If multiple brothers are present, they will all be able to hit the player simultaneously even though it is a single-way combat zone.

Blessed spiders
During the Underground Pass quest, when the player kills Kalrag, the blessed spiders in the same room become aggressive and attack the player simultaneously even though it is a single-way combat zone.

(Chaotic) death spawns
(Chaotic) death spawns summoned by Nechryaels or Nechryarchs in the Slayer Tower are able to attack the player even though it is single-way combat. The player cannot fight the spawns back unless they are not currently being attacked by a Nechyrael or Nechryarch.

Venenatis
If Venenatis is near the single-way combat zone and attacks a player there with her multi-target Magic attack, any nearby players in the single-way combat zone already in combat with another entity will also be hit.