This is a significant feature of the hydra encounter: the PCs have come down an underground river that meets a lava pool, creating five distinct terrain "zones":The environments that PCs will be navigating are likely extra-planar in nature or remote, toxic and/or extremely inhospitable. Create environmental conditions inherent to these environments that are either constantly in effect or use the Condition Track and subject the PCs to things daily. Something as simple/mundane as a low oxygen or extreme humidity environment should be extraordinarily taxing, even for the most well-conditioned.
*The ledge on the non-lave side of the river: safe terrain (if you can fly/climb/teleport up to it), but with lightly obscuring steam between it and the rest of the caern.
*The river, heated by the lava, that inflicts 10 fire damage when you enter or start your turn in it, and carries you 2 squares downstream.
*The cooled lava adjacent to the river, that is difficult terrain and inflicts 10 fire damage when you enter or start you turn on it.
*The lava itself, that is difficult terrain, inflicts 20 fire damage, plus prone and dazed, when you enter it, plus 20 ongong fire damage and dazed (save ends once you get off the lava), plus a DC 17 End check to avoid falling prone if you start your turn on it. If you are adjacent to it, you take 10 fire damage at the start of your turn.
*The solid (higher melting point) rock beyond the lava. This is safe in itself, but is full of salamanders and archons.
*Finally, throughout the cavern except (i) on the safe ledge on the far side of the river, and (ii) in one part of the cavern which has a natural vent in its ceiling, there are fumes coming off the lava that require a DC 17 End check at the start of the turn: failure inflicts 5 poison damage, plus Slowed until the end of your turn.
*The river, heated by the lava, that inflicts 10 fire damage when you enter or start your turn in it, and carries you 2 squares downstream.
*The cooled lava adjacent to the river, that is difficult terrain and inflicts 10 fire damage when you enter or start you turn on it.
*The lava itself, that is difficult terrain, inflicts 20 fire damage, plus prone and dazed, when you enter it, plus 20 ongong fire damage and dazed (save ends once you get off the lava), plus a DC 17 End check to avoid falling prone if you start your turn on it. If you are adjacent to it, you take 10 fire damage at the start of your turn.
*The solid (higher melting point) rock beyond the lava. This is safe in itself, but is full of salamanders and archons.
*Finally, throughout the cavern except (i) on the safe ledge on the far side of the river, and (ii) in one part of the cavern which has a natural vent in its ceiling, there are fumes coming off the lava that require a DC 17 End check at the start of the turn: failure inflicts 5 poison damage, plus Slowed until the end of your turn.
The tiefling PC is enjoying this terrain, but the dwarf fighter is being a bit punished. The drow sorcerer had already set his variable resistance to poison (when checking out the fungi), and is using fire resistance from his Demonskin Tatoo to reduce the fire damage. The invoker has a Book Imp familiar to give him some fire resistance too.
So far only salamanders have been pushed into the lava, and they have fire resistance but still suffer the conditions. But the PCs are planning to use their Fire Horn to strip all the enemies' fire resistance off, and then have a bit of a lava-fest! Which could be amusing.
At this stage I don't think I'll get a PC into the lava, but the hydra - which has some forced movement from a fire breath - has been getting some push into the river, which has also been amusing.