Minecraft wiki entry for Hardcore_mode, associated text: Ghasts. Their fireballs can seriously damage unprepared players, or deactivate portals. Without a flint and steel or fire charges, another fireball must hit the portal to reactivate it.
Falling down huge distances. Even while moving carefully, an explosion can easily destroy the terrain, which may expose a lethally high drop onto blocks or even into lava. Fall damage is one of the leading causes of death in the Nether.
Lava. There are many little pockets of lava inside the netherrack, in addition to the lava seas. Lava is much more dangerous in the Nether because its flow is faster and will flow further. Since water cannot be placed in the Nether, potions of fire resistance are very useful.
The "Basalt Deltas" biome. Avoid these where possible. The jagged terrain, frequent hidden holes of lava with no escape, and the abundance of Magma Cubes, make this biome exceptionally dangerous unless you are prepared with Fire Resistance. Even mining in this biome is dangerous due to the hidden lava. Far too many Hardcore runs have ended in this biome.
Piglins. An attacked piglin or zombified piglin will attract others and they can come at you in droves. You may get overwhelmed, even with good gear.
Beware of the Piglin Brute. Although they only spawn in a Bastion Remnant, they are extremely powerful with 50 health and 19 damage, enough for killing the player without any status effects in a maximum of nine hits when player is stuck (even with Protection netherite armor). They are always hostile, their attacks disable shields, and they will make regular piglins attack you too.
Blazes. Although they're not very challenging alone and in controlled circumstances, in groups of three or more they may present a significant challenge. An unexpected attack of just one blaze can be lethal.
Endermen. Although they are rare, they can be dangerous when provoked due to their teleporting ability and high health, especially when they are accompanied by a blaze or a wither skeleton. They are particularly dangerous in the Nether where you can't use water to counter them.
Wither skeletons. Although they only appear in nether fortresses, this may be the most lethal Nether mob; they are fast, strong, and inflict the wither status effect.