Arcane Bond is a supernatural ability that Daud may unlock in The Knife of Dunwall. This power is specifically applied to another power that Daud can possess: the Summon Assassin power.
Tier I - (0 runes)
Daud already possesses Tier I of Arcane Bond, which gives his summoned assassins access to his Blink and Vitality powers, granting them the ability to teleport and giving them more health.
Tier II - (4 runes)
In addition to the Tier I benefits, Daud's summoned assassins gain the ability to move normally when the Bend Time or Stop Time power is activated. The summoned assassins also acquire the Shadow Kill power, turning enemies' bodies to ash once killed. Summoned assassins can also use Pull on enemies.
Note that the summoned assassins only gain access to these powers and types of support if Daud unlocks the different powers, and how they apply to Daud is how they apply to the summoned assassin.