Unctus's Retrofitter

Multiple instances of toxic are cumulative. For example, if a creature has toxic 2 and gains toxic 1 due to another effect, combat damage that creature deals to a player will cause that player to get 3 poison counters.

Any other effects of that damage, such as life gain from lifelink, still apply.

Effects that modify a creature's power and/or toughness, such as the one created by Blazing Crescendo or a +1/+1 counter, will apply to the creature no matter when they started to take effect.

Conversely, replacement effects that apply to the number of counters put on a player can modify the counters placed this way. For example, Vorinclex, Monstrous Raider's last two abilities can apply to counters placed this way.

If Unctus's Retrofitter leaves the battlefield before its enters-the-battlefield ability resolves, that ability will have no effect. The target artifact won't become an artifact creature at all.

Damage dealt by a creature with toxic grants the same number of counters regardless of how much damage is dealt. Notably, if a replacement effect modifies the damage in some way (such as that of Gratuitous Violence), the number of counters given remains unchanged.