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Hardmode

Hardmode is unlocked by reaching Loop 2 for the first time. This mode significantly increases the difficulty of early game while providing little to no extra benefits. This makes it an obscure mode with particularly poor documentation - thus this guide will only provide a basic summary for the time being.

Runs begin on H1 1-1 which is similar to a L1 start. Enemies that only spawn in loop areas will immediately start appearing in their respective areas, and Elite IDPD immediately start to be able to spawn in. Loop exclusive bosses also start to spawn in H1. Crown vaults will spawn in all valid areas starting from 1-2 H1 and will only give loop crown selections. The only way to use preloop crowns in hardmode is to start with one.

The exception to H1 is that vans will not spawn in right away, which prevents easily unlocking loop crowns through Captain ending. H2 will start spawning in one van, and then each following loop will add one more van as usual. H3 onwards will have exploding Vans and Popo Freaks spawning in, similar to L3.

The weapon pool will not start from loop, but rather with the default starting pool. Weapons will also be added to the pool 1.5x slower, meaning that every 3 portals entered will up the difficulty by 2, while normal mode has a simple 1:1 ratio. A full drop pool in loop is typically filled out by L1 Sewers at the latest; in hardmode this extends all the way out to H2 Labs at the latest.

Every run in hardmode starts with a large weapon chest at the players spawn, which is one of the only upsides. Based on the weapon table in the dropdown this can pull from the first 12 weapons, while enemy drops only pull from the first 9 weapons. As Robot gives +1 difficulty, this also lets them pull from the first 16 weapons in this case.

The weapon pool will not automatically start filled out, but instead H1 must be progressed in order to find better weapon drops. Weapons will appear 1.5x slower than usual, meaning that every 3 portals entered will up the difficulty pool by 2, whereas on normal mode it has a simple 1:1 ratio.

Starting a run spawns the player on top of a large red weapon chest which pulls from the first two pools instead of one, which has a pool of 12 instead of 9 weapons. Robot has +1 difficulty for drops as their passive, which lets them actually pull from the first three pools in this specific case following the drop table.

As weapon drops can be confusing, a guide was made in the NT Discord to illustrate how the drops work:

The only hardmode exclusive content is the Golden Nuke Launcher (gnuke) and Golden Disc Gun (gdisc), which can be unlocked and used in normal mode as well. These start dropping in H1 Palace typically and either will grant the Good Riddance achievement. There is a separate guide on unlock methods and hardmode survival tips.

Bug: on specifically version u99r1, gnuke/gdisc have the Crown of Guns requirement applied to them and won't drop otherwise (typically meant for Gun Gun). This is fixed on the 2021 & 2023 betas on Steam which are u99r2.

There are other downsides to hardmode worth nothing;
Difficulty is increased by 2 a portal instead of 1, which increases enemy spawns. (Difficulty for weapon drops goes slower due to a different calculation.)
Enemies will only drop half their usual rad count, rounded down. This does not effect rad drops from props, Rebel's allies or Plant's saplings. The extra rad from Melting's passive is added on after the reduction.
Ammo has lower chances of being converted to HP, making healing less frequent. The max chance of ammo converting to health is 66% at 0 HP (such as for Headless Chicken), but on hardmode it caps at 50%. In other words the max chance is 2/3rds on normal, or 1/2 on hardmode.