Dragon Magic, page 65.
CALL OF THE TWILIGHT DEFENDER
Conjuration (Summoning)
Level: Druid 6, sorcerer/wizard 6
Components: V
Casting Time: 1 full round
Range: Close (25 ft. + 5 ft./2 levels)
Effect: One twilight guardian
Duration: 1 round/level
Saving Throw: None
Spell Resistance: No
The very ground erupts in a shower of stone and dirt, accompanied by a thunderclap. Standing in the center of the damaged ground is a creature of stone, wood, and plant parts resembling (lie regal form of a guardian dragon.
This spell summons one twilight guardian (see page 121) to fight for you. The spell functions like a summon natures
ally spell, but with one exception. At any time, a summoned twilight guardian can sacrifice itself by simply touching you (a standard action); the twilight
guardian is immediately destroyed, and you gain a number of hit points equal to 1/2 the twilight guardian's remaining hit points before its sacrifice.
Special: A character who learns this spell gains a +1 competence bonus on Knowledge (nature) checks.
Note that there is no cap on how much HP you can get with this spell, nor does the HP expire.
Thanks to Olo Demonsbane for pointing this out.