You have a (+1, +2 or +3) bonus to attack and damage rolls made with this magic weapon.

When dealing damage with this weapon to a living creature, deal an additional (1d4, 1d6, or 1d8) Fire damage.

At +2, Zariel's Spite gains the following:

Can be held and absorb the spell Shadow Blade adding the spell's benefit and damage to itself. You also gain the ability to cast Shadow Blade once per Long rest without expending a spell slot. You can choose to dual wield your Shadow Blade weapon in your off hand instead, chosen when casting the spell. Both weapons gain the Nick Weapon Mastery feature usable only while both spell and Zariel's Spite are in separate hands.

At +3, Zariel's Spite gains the following:

Once per Long Rest, when an enemy is defeated,  fatally or non-lethally, by the wielder, the user recovers a spell slot of Level 3 or lower. If they have not used any spell slots, no additional benefit occurs until one is used.

Wielder gains uses up to their Proficiency of Misty Step, recovered on a Long Rest.

Proficiency with a Rapier allows you to add your proficiency bonus to the attack roll for any attack you make with it.

Notes: those with celestial blood, tainted or otherwise. Aasimar tend to get cursed as part of the attunement., Damage, Combat, Finesse, Vex