Two ways to spend level 12
The breakpoints page is where Extra Attack, Divine Smite, and the slot formula live. This page is one character: paladin mixed with sorcerer, from the first level.
Both splits take paladin far enough for Divine Smite. Smite arrives at paladin level 2 and spends a spell slot.
Six paladin levels and six sorcerer levels keep Extra Attack, which paladins learn at level 5, and Aura of Protection at paladin level 6. The aura adds your Charisma modifier to saving throws for you and nearby allies. Sorcerer level 3 is the first pair of metamagic picks. Sorcerer level 6 adds your Charisma modifier to the damage of spells that match your draconic element.
Two paladin levels and ten sorcerer levels keep smite, heavy armour, and a fighting style, then spend the rest on sorcerer. That split stops before Extra Attack and before the aura. The slots go higher, which is more fuel for smites, and sorcerer level 10 offers another metamagic pick.
What to take on a first run
On a first run, take paladin levels through 5 or 6 before any sorcerer level. Act 1 is kinder with a second attack, and level 6 adds the aura. Withers can rebuild the two-and-ten split later for 100 gold, from level 1, with your gear still in the bags.
Take Oath of Vengeance. Vow of Enmity spends Channel Divinity and a bonus action. Mark one enemy. Your attacks against that enemy have advantage for the duration on the tooltip.
One spread the character creator can hold: Strength 16, Dexterity 10, Constitution 14, Intelligence 8, Wisdom 10, Charisma 16. If you will take the hag's hair, start Strength or Charisma at 17 so the +1 moves the modifier.
Draconic bloodline. Each sorcerer level adds 1 hit point. The unarmored armour number applies while you wear nothing. A paladin's armour replaces that number and keeps the extra hit points.
At paladin 4, take Great Weapon Master or raise Strength. Great Weapon Master subtracts 5 from the attack roll and adds 10 damage on a heavy melee hit. Leave the toggle off when the armour is high. Savage Attacker rerolls the melee damage dice on a hit and keeps the higher roll, and the smite dice on that hit are in the reroll.
A 6/6 character gets a second feat at sorcerer 4. A 2/10 character takes sorcerer feats at sorcerer 4 and 8, and never takes the paladin feat, because paladin stopped at 2. The breakpoints page is where that feat rule is written out.
One turn
You have one bonus action. Vow of Enmity and a quickened spell both want it. Use the vow on the turn you need advantage on every swing. Use Quickened Spell on the turn you need Hold Person or Haste and still want to attack.
Open the reactions menu and set Divine Smite to ask. Spend the slot on a critical hit, or on a hit against a creature that is held or paralyzed and within reach. A melee hit on a paralyzed creature within reach is a critical hit when it lands. An ordinary hit can stay a weapon hit.
Shield spends the character's Reaction resource when it is confirmed. Divine Smite's post-hit prompt spends a spell slot but does not spend that resource, so it can still smite even after Shield used the turn's reaction.
Bless, Haste, and Hold Person each take concentration. One of them is active at a time.
A broken oath blocks respec until the oath is restored. The Withers page has that gate.
Shield costs a Reaction point and a level 1 spell slot. The Divine Smite post-hit reaction prompt costs a spell slot but no Reaction point.
Divine Smite can trigger on multiple melee hits within a turn, including Extra Attack or dual wielding.
The shared reaction-resource reference identifies Divine Smite as an exception that does not expend the Reaction resource.
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