Sol Heredit is a melee duel. One boss, no minions, no adds. The whole fight is about prayer timing and floor positioning, and it gets harder in six clean steps.

Getting there

Two ways in:

Teleport nexus. Find Sol Heredit in the boss teleport list, described there as the final fight of the colosseum. The nexus shows its notable drops before you commit.
Command. ::solheredit takes you straight there.

The command puts you in your own private instance and does not cost an instance token, so you can run it as often as you like.

The nexus lists Sol Heredit as Master difficulty with no entry requirements.

The six phases

Sol announces every phase change out loud. Use the quotes as your cue rather than watching his health bar.

Phase	Starts at	He says
1	100%	—
2	90%	"Not bad. Let's try something else..."
3	75%	"Impressive. Let's see how you handle this..."
4	50%	"You can't win!"
5	25%	"Ralos guides my hand!"
6	10%	"LET'S END THIS!"

Every phase change from 2 to 5 spawns another patch of molten sand and another prism. The hazards accumulate, so the floor gets progressively more dangerous while the boss itself gets faster.

He speeds up

Phase	Attack speed
1	10 ticks
2	10 ticks
3 onward	8 ticks

The combo attack

This is what kills people, and it is the one mechanic you must learn.

From phase 2 onward, roughly one attack in eleven, Sol uses a three hit combo. There is a 30 tick cooldown so it will not chain back to back.

The combo turns off your Protect from Melee. On every tick leading up to the hit, if Protect from Melee is active it is deactivated, right up until the final tick before damage lands.

What that means in practice

You cannot leave prayer on and tank it. You have to re-activate Protect from Melee on the last tick, just before each strike connects. Get it right and you take nothing. Get it wrong and each strike hits:

Strike	Phases 2 to 3	Phase 4 onward
Second	25	30
Third	35	45

A fully missed combo in phase 4 or later is 75 damage from two strikes alone. That is the difference between a kill and a death.

Floor hazards

Molten sand

Patches of burning sand on the floor. Standing on or beside one deals 6 to 8 damage repeatedly for as long as you are there.

Individually small, but it stacks with everything else and there are more patches every phase. In phase 6 a new patch spawns every 10 ticks continuously, so the floor keeps filling up while you finish him off.

Prisms

A prism spawns at each phase transition and walks between the corners of the arena, firing a beam of light. The prism charges for a set number of ticks before the light fires, and that window shortens as the fight goes on.

What to do: watch where the prism is walking and stay out of the line its beam will take. With multiple prisms up in the later phases this becomes the main thing dictating where you can stand.

Putting it together

Fight him in an open part of the arena. You need room to step off sand and out of beams.
Keep Protect from Melee up for normal attacks.
Combo attack → re-flick Protect from Melee on the final tick before each strike.
Step off molten sand the moment it lands under you.
Track the prisms and stay out of their beam lines.
Phase 6 is a race. Sand spawns constantly, so kill him rather than trying to play it safe.

Combat achievements

Sol Heredit Grandmaster requires 250 kills. Since the fight costs no token and runs instanced, this is purely a time investment.

The fight also tracks two things that suggest restriction tasks: whether you used a weapon other than a spear, and whether you turned run on at any point. If you are going for those, use a spear and walk the entire fight.

Your kill time is tracked too, so there is a speed task to chase once you are comfortable with the combo timing.

What to bring

Melee. It is a melee duel and he follows you closely.
Prayer potions. Constant flicking on the combos burns prayer faster than the fight length suggests.
Food that heals fast. Chip damage from sand plus a missed combo adds up quickly.
A spear if you are chasing the weapon restriction task.

Related

Smol Heredit is the pet and gives +2% Strength experience. Sol Heredit is also one of the bosses available through instance tokens if you want to bring someone with you.