The creature is the source of the new damage.

The delayed triggered ability Arcbond creates will trigger even if the target creature is dealt lethal damage. For example, if a 3/3 creature is targeted by Arcbond and later in the turn it blocks a 7/7 creature, the 3/3 creature will deal 7 damage each other creature and each player.

Damage dealt by the creature because of the ability Arcbond creates isn’t combat damage, even if it was combat damage that caused the ability to trigger.

If two Arcbonds have resolved targeting different creatures, and damage is dealt to one of them, the delayed triggered ability will cause that creature to deal damage to each other creature and player. This will cause the delayed triggered ability from the other Arcbond to trigger. The second creature will then deal damage to each other creature and player. This will continue until one or both of the creatures die or the game ends. If this doesn’t happen (perhaps because both creatures have indestructible and damage dealt to the players is being prevented), and no player breaks the loop, the game will be a draw.