Let's Look at the Colossus!

 As indicated by the Scroll of Titan Summoning in the DMG, there are several new Titans to join the Tarrasque, Kraken, and Empyrean.

As someone who loves mixing fantasy with science fiction, I've got a soft spot for Constructs. The Colossus has some perhaps unexpected lore - they are created by devout followers of a deity, and if the artisans who spend decades crafting these things please said deities, the crystals inside might thrum with divine magic that brings them to life.

Generally speaking, though, these are typically found as relics of some ancient past. Personally, I think you can ditch the divine element here if you want - it explains a resistance to Necrotic and Radiant damage and the radiant attacks it has. But I think you can explain both its Radiant Ray and Divine Beam as elements of long-lost technology as well.

Like the Warforged Colossus from Eberron: Rising From the Last War, this is a Gargantuan CR 25 Construct. There are similarities, but the stat block here is actually a bit simpler.

The Colossus has an AC of 23, 553 HP, and a 60-foot movement speed. It has pretty decent saving throw bonuses except for Charisma and Intelligence. Like golems, it has an immutable form and immunity to poison and psychic damage.

The Colossus makes three attacks, the attacks being a Slam and a Radiant Ray. These are just simple melee and ranged options. Both have a +18 to hit - so you can feel pretty confident that you'll connect with them (even my Ravnica game's Babrarian, who has an AC of 25, would be hit more often than not by this thing). The Slam deals 32 average damage and pushes the target up to 20 feet away from the Colossus. It also has a reach of 20 feet, so you could potentially knock an adventurer back far enough that they can't even get back into melee with you on their next turn. The Radiant Ray has a range of 300 feet and hits for 22 radiant damage, and also knocks Large or smaller creatures prone.

Now, while that's likely 96 damage in melee or 66 at range (and again, very likely to hit,) the real terror that this thing unleashes is its Divine Beam, which is a 300-foot-long, 10-foot-wide line AoE attack. Creatures within make a DC 26 Dexterity saving throw, taking an average of 65 radiant damage on a failure, or half as much on a success. Creatures reduced to 0 HP by this are disintegrated (even on a success,) leaving behind only their magic items. This ability recharges on a 5 or 6.

I want to emphasize that DC - at 26, even a character who maxes out their Dexterity won't be able to make this save unless they also have proficiency in it, or are otherwise boosted by things like Bless, Aura of Protection, or the like.

Its legendary actions are simple - it can shoot a Radiant Ray or it can do "Stomp," which allows it to move half its speed (so 30) without provoking opportunity attacks and make a Slam attack at any point during said move. Thus, essentially, we're really looking at 6 attacks per round, if not using the Divine Beam.

So, how do we use this?

Well, as a Titan, this is really a climactic fight. I think you want to build up to this. A couple possibilities present themselves:

A villain's plan might primarily be to awaken one of these things, taking advantage of its being from a bygone era to set it against innocent targets. The Colossus could have been built for an evil deity, but even if it's built for a good one, maybe there's some city that grew up around an ancient holy site, and no one for thousands of years has realized that technically the site was supposed to be left as it was.

Another possibility is that the Colossus guards some profoundly sacred temple or even a dangerous site holding some powerful and destructive relic, and the Colossus is meant to keep it from falling into the wrong hands.

For me, I have a region in my homebrew setting where the ancient robots from a fallen civilization roam in an endless, misguided war. The Colossus could be the pinnacle of the "Mechanicos" that continue fighting like this. In this case, the Radiant damage that the Colossus inflicts are probably super-advanced laser technologies, with the Divine Beam being a kind of extra-powerful nuclear cannon.

At CR 25, the Colossus is worth 75,000 xp. If we have a group of 5 player characters, that means it's taking up 15,000 xp per player. This makes it north of High difficulty at level 18, and puts it between High and Moderate difficulty for both levels 19 and 20. In other words, this is never going to be an easy fight (it's just south of moderate for level 20 if you have 6 players).

This should, assuming the encounter difficulty is properly calibrated, be a solo monster. I really don't see a fight with one of these needing or benefiting from having any other things to fight.

Or...

Well, here's the thing - a Colossus is simple enough to run that I actually think you could very easily have a fight in which the players are on the same side as the Colossus. The key here, though, is that the enemies would have to be truly staggering to make such a fight anything other than a cakewalk.

There's not, as far as I know, any real guidance on how to adjust encounter difficulty when the party has NPC allies. I don't know if it's balanced, but I tend to just add the monster (their CR or their XP, depending on my encounter-building guidance - which has tended toward XP since the new DMG came out) to the budget. In other words, adding a Colossus to the party's side would mean adding an equivalent amount of XP worth of monsters to the enemy side.

And given that that's 75,000 xp, that means we could fit in, for example, three Balors (and change) on top of what else the party winds up fighting.

One thing that I think really works for Titans as foes is the notion that these are such massive creatures that they might be, essentially, more than one fight. Titans have a tendency to rampage across some wide area - this is the MO at least for the Tarrasque, Blob of Annihlation, and Elemental Cataclysm, and I think a Colossus could easily be set on such a path. Thus, it might actually make sense for the Titan to simply fight to defend itself while pushing forward to some goal. The party might then have the opportunity to retreat and rest before catching up with the Titan and continuing the fight.

Perhaps more than any of the other Titans, the Colossus is pretty straightforward to run. I think you can throw this at a high-level party (again, probably never lower than tier 4 unless they have significant NPC help) and trust that it's going to take a lot out of them.