In Challenge Mode (basically hard mode), the Twin Furies gain an additional 100,000 life points (and combat stats), a few new abilities and slight alterations to their normal abilities.
- Avaryss' Wall Charge ability deals slightly more damage, and will also drop the player's protection prayers if they are hit by it.
- The Twin Furies channelled bomb fire blasts deal double damage from normal mode.
- Throughout the fight, a red fireball will bounce around the arena. Its impact zone is shown with green arrows (similar to Detonate, although a glitch can occur where this may not appear). If the player is not at least 3 squares away from the impact zone, they will be dealt with up to 3,500 damage.
- After the channelled bomb, the Twin Furies will unleash a red and blue fireball into the arena. The player should follow the blue fireball while avoiding the red one. Both fireballs bounce three times before disappearing. The player needs to get hit by the blue fireball, which will then unleash two blue fireballs on the Twin Furies for up to 6,500 magic damage each. Otherwise, if the player avoids it, the red fireball will hone in on the player after three bounces for up to 6,000 damage each.
Players should still follow the same strategy as in normal mode, as their damage is still low enough to be offset by Soul Split and the Vampyrisim aura/scrimshaw. The player should also avoid getting hit by the red fireballs as getting hit too much by them can easily deplete the player's food supplies during that kill.
At the fourth ability, players should run to the blue fireball when possible. This does no damage to the player and will send two blue fireballs back at the Furies for up to 6,500 magic damage each, which can significantly shorten the kill. Don't ignore it or the player will receive a massive hit which can potentially kill them if they are subjected to other attacks.