There should be reward for procing enemy's Guardian Angel.

If it going to give armor and death immunity, it make assassin afraid to engage enemy adc cuz the risk is too high and there are no reward at all. Which cause a problem that noone will focus enemy ADC which give them too much free time to deal damage. Instead of giving direct AD, this is what should be done:

An angel aim for you in battle, grant bonus AD + some bonus, upon taking lethal damage, the angel will take damage for you, knock you back 10 space, make you invulnerable and restore health for 4 seconds. The angel will return when you back to base or item off cool down, but she/he(male champion gain a female angel, except for ) will only take lethal damage for you after the item is cooled down.

The cooldown of this item must be less, it give you life for some price, and the price for killing the angel is the deal with the devil Blessing of the devil: The angel's killer grant a small aim for him in battle for 10 seconds, attack with him in combat and deal bonus damage on hit.