Attacking a mob while a certain Rune is held in the offhand will give the sword an additional ability. Doing this consumes 5 of that Rune, and lose 2 durability instead of 1. List of different Rune Effects:
Charged Rune: Spawns particles on hit. This effect is purely visual.
Fire Rune: Ignites target on hit, dealing 4 (×2) total fire damage.
Poison Rune: Inflicts the target with Poison II for 3.6 seconds.
Water Rune: Inflicts the target with Slowness I for 3 seconds.
Wind Rune: Deal extra knockback on hit.
Wither Rune: Inflicts the target with Wither III for 2 seconds.
Runic Sword can be obtained as a drop from the following mobs: