I had actually designed a really similar Eldritch spell for Heroic Fantasy Handbook. Just as Ary suggested, it was built as a mult-effect spell.
Boil Blood Range: 150’
Eldritch 5 (Grey) Duration: instantaneous
This spell can boil the blood of a living creature, thereby inflicting terrible wounds and great suffering, and possibly causing it to explode like an overheated furnace. The target must make a saving throw versus Blast. If the save fails, the creature suffers 8d10 points of damage and is stunned during its next initiative. If the save succeeds the creature suffers half damage and is not stunned.
In either case, if the target is reduced to 0 hp or less by the spell, it detonates in an explosion of boiling blood, scalding puss, and melted organs. The explosion kills the target instantly. Each creature or object within 5’ of the exploding target suffers 1d6 points of damage, or half that on a successful saving throw versus Blast.
I built it as:
Boil Blood (5): Blast, 1d8 damage per level (35), maximum 8d damage (x0.95), elemental – fire (x1), target 1 creature (x1), range 150’ (x1), duration instantaneous (x1), saving throw reduces spell effect by half (x0.75), eldritch (x1.5), cost 37.4;
---- this is the base effect, dealing 8d8 damage to the target with a save for half
and nausea-inducing (10), elemental – fire (x1), target 1 creature (x1), range 150’ (x1), duration instantaneous (x1), saving throw avoids spell effect (x0.5), eldritch (x1.5), cost 7.5;
--- this is the nausea component, which causes a round of nausea if the save is failed
and 1d6 damage per level (27), maximum 1d damage (x0.1), elemental – fire (x1), target 10’ diameter sphere (x1.25), range 150’ (x1), duration instantaneous (x1), saving throw avoids spell effect (x0.5) [original creature], saving throw reduces spell effect by half (x0.75) [secondary blast], eldritch (x1.5), cost 1.89;
--- this is the secondary explosion. I assumed that the conditional explosion would occur if the original target failed its save, but would not occur if the original target failed its save. therefore, the secondary explosion has two saving throw modifiers - x0.5 for the original creature to avoid it entirely, then x0.75 for the creatures hit by the secondary blast to reduce damage by half.
total cost 46.79
--- note that eldritch spells have a 1.5 source modifier for blast spells; as an arcane spell this would be 31.2 points, or 4th level.