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