Is my Paladin stacking Smite spells correctly?

Question

I am the DM for a level 2 Paladin. Today, he managed to pull off what the table found to be a very powerful, yet simple, combo. The two of us read over the rules and we both think it was by the book, but I'd like to be sure.

The paladin first cast Thunderous Smite as a bonus action on his longsword, then attacked an owlbear as his action. He hit, so he added 2d6 damage to his regular 1d8+STR and incurred a STR save (the results of which are tangential to this question). He then spent his second (and final) spell slot on Divine Smite to add 2d8 radiant damage. The final damage total was 3d8 + 2d6 + STR.

In a later battle, after resting up, he tried this maneuver again and rolled a natural 20 on the attack. Since crits double damage dice, his final total was 6d8 + 4d6 + STR damage.

Is this interpretation of how the Divine Smite class feature interacts with the Smite spells correct? Nobody at my table has a problem with the balance, however it was so comically strong of a single attack that we had doubts of its legitimacy.

Are we missing something here, or is this the correct amount of damage to deal for the presented situations?

Answer 1

This is a perfectly valid use of Divine Smite and a smite spell

We can see in the description for Divine Smite that the attack must be a melee weapon attack:

Starting at 2nd level, when you hit a creature with a melee weapon attack, you can expend one spell slot to deal radiant damage to the target, in addition to the weapon’s damage. [...]

We don't have any additional restrictions on the use of Divine Smite, other than that we must have a spell slot to expend.

The various Smite spells available at time of writing (Banishing, Blinding, Branding, Searing, Staggering, Thunderous, Wrathful) all begin with a variant of the following:

The [next] time you hit [a creature] with a melee weapon attack...

There is not a restriction saying you cannot combine these two effects. So, given they meet all requirements (a melee weapon attack and a spell slot to expend), a paladin could indeed use both a smite spell and the Divine Smite feature on a single attack.

We can also look in the Sage Advice Compendium for further clarification:

Can my paladin use a smite spell along with Divine Smite? As in, I cast wrathful smite, hit, then use Divine Smite on the same attack?

Yes, you can use Divine Smite on the same weapon attack that benefits from a smite spell, such as wrathful smite—as long as the attack you make after casting the smite spell is a melee weapon attack. Divine Smite doesn’t work with any other kind of attack.

While this is certainly powerful, it is also resource-intensive. A low-level paladin choosing to do this might struggle to perform during the rest of the adventuring day.

Answer 2

This is correct, but note how much of his available resources this character is using up for each of these attacks. If there's another battle before he gets a long rest, he's not going to be nearly as impressive.