Okay. So let's have some actual abilities and average numbers.
5E's assumed party of four. Which four? Let's go classic Coke. Cleric, fighter, rogue, and wizard. Everyone's 10th level.
Base hit points (ignoring CON mods) for those characters would be (minimum/average/maximum): Cleric 17/53/80, fighter 19/64/100, rogue 17/53/80, wizard 15/42/60. Of these the cleric and fighter are the only two to likely have a CON mod big enough to matter. In reality their HP would likely be somewhere between the average and max.
So, one thing jumps right out. If facing off against the average HP party and Soth opens up with that necro-fireball, dealing average damage, and if everyone fails their saves...that's almost a one-shot TPK. The cleric and wizard will likely fail their DEX save. The fighter might make their save, but probably not. The rogue certainly will. It's 1/2 or nothing for the rogue. Still, an opening salvo of 35-70 damage is going to hurt. And a lonely 10th-level rogue isn't going to win against Soth.
Soth's power word kill deals 50-100 damage. Which, even if the targeted PC saves against, will take any one of these down...unless they're at full average HP at least. Which, given the context of the actual encounter with Soth in the module, is very likely impossible.
Soth has a +12 to-hit and 3-4 attacks per round. At 10th level the party can expect to have 1-2 PCs with higher than 20 AC, maybe 2-3 depending on the cleric's domain. The wizard will likely have shield, so can burn a few slots to maybe avoid being hit. But when Soth hits, he's dealing an average of 28 damage and that rider prevents healing for a round. Three hits on any one of the PCs is enough to take them down. Only two on the wizard. And that no-healing-for-you rider is a killer.
Soth's movement legendary is beefy. Especially since you can move between attacks. Move, knock prone, attack prone target with advantage...move, knock prone, attack prone target with advantage. Regular move of 30ft, three legendary moves for another 90ft, anyone within 5ft of his 90ft of legendary movement will likely be knocked prone (and nothing says he can't zig-zag to get multiple passes on the same target), and he can pepper in his regular movement and attack the prone targets. All the while getting advantage on prone targets, not provoking, and prone targets having disadvantage on attacking him.
And that's not even touching his spellcasting: command, hold person, dispel magic, and banishment.
Soth has both magic resistance and legendary resistance, so spells are unlikely to really affect him.
A referee playing him even halfway smart would wipe the floor with even a fully rested 10th-level party. Soth has a few great openers. 1) Necro-fireball the party, legendary move, legendary attack the cleric. If that combo doesn't outright kill the cleric, the cleric will die next round when Soth focuses on the cleric until they're dead, then spread the love. 2) Power word kill the cleric, legendary move, legendary attack the cleric. Again, if they're not already dead, they will be next round after the necro-fireball. 3) Either of the above only ignore the cleric and let them waste time and slots trying to heal the party. 4) Legendary movement, legendary attack, regular movement, and multiattack to hit everyone in the party with that no-healing-for-you attack. Then necro-fireball the group. A well-timed banishment, hold person, or command would also wreck the party.
So looking at the actual abilities and some average numbers it looks like in two, maybe three, rounds Soth would TPK the party. For the PCs to even have a chance they'd need to be power gamed to the hilt and avid wargamers who coordinate and strategize in their sleep.
