In mission 1, we no longer need to lead Penny back to the entrance, and Clockwork King spawns in the end room with her, so there's no need to go back to the entrance at all.
In mission 2, the Riders are buffed and colour-coded. See the Enemy Changes stuff below for details.
In mission 3, the sisters are in specific locations - Infernia near the start, and Glacia at the crossroads right before Dra'Gon. Also, the six other psychic hostages who normally don't matter, matter on Hardmode. Failing to rescue them, empowers the bosses at the end. See the Enemy Changes stuff below for details. Lastly, we don't need (nor have the option, it seems) to lead anyone out - Infernia and Glacia will just join up and follow us (and probably die trying).
In mission 4, the mission plays out basically as usual, though there's no need for Essence of the Earth.
Mission 5 is the big one. The run to the end is exactly as normal, but once we get to the final room, the first difference is immediately apparent - we can't attack the portal generator things, so we can't try to spawn the Honoree early. Instead, we have to attack Hro'Dtohz until 50% as usual. But instead of the Honoree coming through the portal to join us in the fight, we will get sucked through the portal, and deposited onto a Rikti Mothership intact on the Rikti Homeworld.
My understanding of things here is pretty fuzzy still at this point, but the basic idea is that we need to move downwards through the facility to the final room where we'll re-confront the bosses. There's some generator things to destroy along the way, and I genuinely don't know what the hell they do. Once we get the bosses defeated, the reactor goes critical, and we have to flee the Mothership back out the way we came.
Also, it's worth noting that while inside the Mothership, Temporary Powers are disabled. This includes prestige stuff like ATT, so no using that to get people around. Once one person exits the Mothership, though, that person can just ATT everyone back to our own universe, and we're fine.
The Rikti are broadly the usual - decent against psi attacks if they're armoured, lots of swords and energy weapons. Some of their goons have had their AI flip to one that's tagged "Melee_NeverAfraid", which I think means they won't run away, maybe from AoE damage patches? Not sure. There's also a bunch of places where enemies with one attack have two, enemies have faster recharges or more accuracy, etc. Some of their bosses also have versions of Assault/Tactics, depending on how high the difficulty is set, and the overall faction has a decent bit of extra confuse powers. The inclusion of War Priests in the regular spawns also adds extra pain - they combine psychic attack powers, the good parts of the Empathy powerset, and Fulcrum Shift.
The Guardians, their Rad Defender things, have been upgraded to Lieutenants, and now have better powers. The heal is an auto power that fires off every 8s, though hold/sleep/stun/endurance drain turns it off. Accelerate Metabolism is slightly stronger, Protection Shield is weaker, but much more anti-toxic, and lasts longer, and they now have Radiation Infection as well.
One thing worth noting is that the Communications Officers spawn "Volatile Porta"s, which summon Hydra enemies instead of Rikti goons. Each portal spawns a tentacle after 3s of existing, and then a Kraken after 30s. When it spawns the Kraken, it also closes. The portals are incredibly difficult to destroy with damage, and instead should be closed by hitting them with Holds, Sleeps, Stuns, or Fears - ten magnitude of any one of these, will instantly kill the portal.
These are the new EBs that get casually sprinkled into the spawns throughout. Their core loadout is the exact same powers and stats of the Heavy Assault Suits we've all seen before - lasers, fusion weapons, and 20% resistance to all damage, with an extra bit for psi/tox. However, they have some extra tricks going on. First, they use Teleswap, which teleports a player (or one of our pets or whatever) to their location, hits them with a brief Mag 20 confuse, and warns everyone with a popup message. Second, even if you hit 'em with enough anti-fly to ground 'em, they have actual good movement on the ground. In exchange for all this, they're oddly vulnerable to teleport, so even at EB rank and with high difficulty, they can be TP Foe'd or Fold Space'd easily.
The more important thing is that when they die, they spawn a Neural Nullifier. The nullifier is a large, visible bubble. Inside it, we take -80% defense, -120% resist(all), Mag 10 fear, and minor unresistable damage over time. This stacks, so standing in multiple is a rapid death sentence, but even just one can cause tanks to absolutely fall apart under any remaining enemy fire. Most of the time, the best solution is to kill the SAS last, which is easy since they're EBs. When multiple groups hit in a row (or sometimes at once), though, it's easy to end up with a bunch of these on the battlefield all at once, causing several deaths to happen in quick succession. MMs can rest easy, though - the bubble only affects Players, not pets!
In Hardmode, this guy gets the appearance of the Clockwork King from dimension Epsilon Tau 27-2 (Clockwork Earth), and his powers get significantly upgraded. His damage resistances aren't as potent, but he's got Challenge AV health (more than double normal AV health), so that's mostly irrelevant. He gets Psi Blade/Greater Psi Blade, lots of stuff with Knockup, Psychic Wail, and most obnoxiously, a proper Tanker-style Taunt, complete with the -75% range in the full AoE. Overall, a lot stronger than the old version, but still basically just a big guy with HP and damage.
Related, all the Clockwork in this fight are slightly stronger versions of the Psychic Clockwork faction. Honestly, most of them are literally unchanged from the normal Psychic Clockwork.
Mostly the same, but stronger. Pestilence, e.g., has Toxic damage added to its Laser/Fusion weaponry, all its DoT changed to Toxic, and does more -def with every attack, even the ones that didn't do -def. Most of their signature stuff (Pestilence's Envenom, Death's dark-themed powers, War's buffs) are utterly unchanged, except for the big overall change that the riders are now AVs in hardmode, not EBs. Oh, and Famine's powers that do all the -end, -recovery, and -recharge, have been changed to cold damage instead of toxic. Not a big deal, though, since they're otherwise exactly the same BS as always. Also, because they're a bunch of Assault Suits, the riders have the Neural Nullifier thing when they die.
Like Pestilence's toxic/-def above, Death has negative/-tohit, War has fire/extra DoT, and Famine has cold/slow. In addition, each has a vulnerability. Pestilence takes extra psychic damage, Death extra energy, War extra cold, and Famine extra fire.
The new version (hardmode or not) has nerfed Hamidon - he no longer deals electrolytic damage, so EoEs are no longer needed, nor even helpful. He just does normal toxic damage.
The entire fight in the bowl - Hamidon, Mitos, Pylons, etc. is exactly the same between normal and hardmode, except for the generic Rikti goons having all the general buffs. The only real hardmode change that matters, is the dropship.
The dropship is a big tanky flier that will circle the bowl. It's possible to destroy it (it's basically just a challenge AV with 20% resistance to non-psi damage), but not really worth the effort most of the time. Every 3s it will fire a beam at the closest target, blasting someone for around 1500 damage, dealt in three separate DoT ticks of 500-ish each. Additionally, it will summon Rikti goons, and drop bombs onto the battlefield.
The bombs work like the normal UXBs from the invasion zone event, but with a twist - if the bomb goes off, it creates a Neural Nullifier zone, same as the dead Assault Suits.
I haven't yet had time to parse through this guy.
I haven't yet had time to parse through this guy.