Kyuss is a complex and impressive opponent, but considering the comments others have made, he will spend his short freedom mostly casting harm on himself. He deserves better. Many have suggested increasing hitpoints, adding minions, or gating in meanies, all of which are viable options. But I'd like to discuss another option: Changing feats, and adding epic spells.
With 30HD, Kyuss can have 4 epic spells, and has taken two already (improved spell capacity, twice). Maximize Spell and Extend Spell, on the other hand, are largely unused in his spell selection (and he has little to benefit from those, even if rewriting spells known). I'd rather give him those:
1) Multispell - cast two quickened spells per round. He has plenty of them.
2) Epic Spellcasting - he can now cast three epic spells per day (spontaneously). That he has spent 1500 years trapped in a monolith gave him enough time to develop some.
If only he had one epic feat more; I'd definitely give him Spell Stowaway (Time Stop)...
Now for the epic spells (Maximum Spellcraft DC for automatic success if taking 10: DC62)
SPAWN THE WORM
Spellcraft DC: 62
Componets: V, S
Casting Time: free (counts as quickened spell)
Seed: Animate Dead DC23
Factors: cast as quickend (+28), change from touch to ranged touch (ad hoc +4), create up to 25 HD (+5), favored spawn of kyuss (ad hoc +2).
Kyuss points at a corpse, and as a stream of worms fly towards it, they transform it into an undead menace. The creature rises as a favored spawn of kyuss (template in Dragon 336). Note that he can control the creature, as his CL for evil necromancy spells is 25. He also can use his Familiar ability on it.
Spellcraft DC: 61
Casting Time: sa
Range: 300 ft.
Save: Fort DC41
SR: yes (d20+27 to overcome)
Seed: Slay DC25
Factors: cast as standard action (+20), increase save DC x3 (+6), gain +1 bonus on CL check to overcome SR x3 (+6), no verbal component (+2), no somatic component (+2)
Kyuss glances at you, and his divine will crushes your life force.
The target dies instantly. If it makes its saving throw, it takes 3d6+20 damage instead. Death Ward protects from this spell (so Kyuss will cast ABATE DEAD only after successfully dispelling death ward. Any ally with Mordenkainen's Disjuction would come in very handy, too).
BLAST OF UNLIFE
Spellcraft DC: 59
Components: V, S
Casting Time: sa
Area: 20-ft. radius
Save: Fort DC36
SR: yes (d20+20 to overcome)
Seed: Heal DC25
Factors: cast as standard action (+20), change from touch to target (+4), change from target to area (+10).
As Kyuss bashes his wormy fists together, millions of shadowy worms explode outwards.
All living creatures in the area are reduced to 1d4 hitpoints unless they make a fortitude saving throw. Undead (and creatures with negative energy affinity) are instead restored to full health.
(This makes for a much more effective healing mechanism than time stop + harm, harm, harm, harm. Also, may heal allied undead such as those created by Spawn the Worm)