The overwhelming heat weakens your bodily functions. You need to look for equipment to protect you from the heat.
Heat Exhaustion is a debuff caused being in The Underworldbiome in Death Mode without any form of Heat Immunity. It is initially harmless while active, but will increase in severity over time. The following debuffs are inflicted onto the player while this debuff is active and are cumulatively built up:
The timer is raised twice as fast while the player contacts lava. The debuff is removed as soon as the player exits The Underworld or obtains Heat Immunity. However, the additional debuffs will slowly be removed over time rather than instantly.