The elves left a guardian to watch over the widget. The guardian might be a construct, an elemental, or some type of unique good undead. But since the elves thought "Well, we want to be able to get the widget back in 300 years" they would have left some mean of bypassing the guardian without actually killing it.
Maybe the guardian asks a riddle (in elvish) that relies on obscure clues. Characters can make an appropriate knowledge check to understand the clues well enough to begin solving the riddle. This will reward players with knowledge ranks.
Try this: Use the Magical Construct Template from Monte Cook's site.
www.montecook.com
Apply the template to a sphinx or a pixie.
Creating a Magical Construct
"Magical Construct" is a new monster template you can add to any corporeal creature that is not of the shapechanger, elemental, or ooze type -- hereafter referred to as the "base creature." The resulting creature is of the construct type. The material used to make the construct (stone or metal) helps determine its powers and abilities.
A magical construct uses all the base creature's statistics and special abilities except as noted here.
Hit Dice: Increase to d10 (if the base creature has d12 HD, do not reduce the total).
Speed: Constructs have 75% of the speed of the base creature, unless the speed is for flying; in that case, the speed is 50% of the base creature and the maneuverability falls to Poor.
AC: Natural armor increases by +8 if the construct is made of stone, +12 if it is made of metal.
Attacks: The magical construct retains all the attacks of the base creature.
Damage: The magical construct retains the damage values of the base creature.
Special Attacks: A magical construct retains all the special attacks of the base creature, except those that involve changing shape (something the magical constructs cannot do). It also gains one of the following.
Breath Weapon (Su): First or second round of combat -- cloud of poisonous gas, 10-foot cube directly in front of the construct lasting 1 round, free action every 1d4+1 rounds; Fortitude save (DC 17), initial damage 1d4 temporary Constitution, secondary damage death. Available to metal constructs only.
Breath Weapon (Su): Turn to stone permanently, cone of gas 60 feet long, every 1d4 rounds (but no more than five times per day); Fortitude save (DC 17).
Breath Weapon (Su): Sleep gas cone, 60 feet, every 1d4 rounds (but no more than five times per day); Fortitude save (DC 17) or fall asleep for 1d10 minutes.
Slow (Su): The construct can use slow as a free action once every 2 rounds. The effect has a range of 10 feet and a duration of 7 rounds, requiring a successful Will save (DC 13) to negate. The ability is otherwise the same as the spell.
Haste (Su): After it has engaged in at least 1 round of combat, the construct can haste itself once per day as a free action. The effect lasts 3 rounds and is otherwise the same as the spell.
Special Qualities: A magical construct retains all the special qualities of the base creature and also gains the following:
• Immune to mind-influencing effects, poison, disease, death affects, paralysis, stunning, sleep, and similar effects.
• Not subject to critical hits, subdual damage, ability damage, energy drain, or death from massive damage.
• Darkvision 60 feet
• Destroyed when reduced to 0 hp, cannot be raised.
Saves: Same as the base creature.
Abilities: Modify the base creature as follows: Str +10, Dex -4, Con [No score], Int [No score], Wis -2, Cha -10 (minimum 1).
Skills: Constructs have no skills.
Feats: Constructs have no feats.
Climate/Terrain: Any land and underground
Organization: Solitary or gang (2-4)
Challenge Rating: Up to 10 HD: as base creature +3 (+4 if metal); 11+ HD: as base creature +2 (+3 if metal)
Treasure: None
Alignment: Always neutral
Advancement: Same as the base creature
Construction
The cost for each construct includes that of the physical body and all the materials and spell components that are consumed or become a permanent part of it. It amounts to 10,000 gp per HD.
The first task is carving or assembling the construct's physical body. The creator can assemble the body or hire someone else to do the job. The builder must have the appropriate skill, which varies with the construct.
The real work of creating a construct involves extended magical rituals that require two months to complete. Understanding the rituals requires a character of the appropriate level with the Craft Magic Arms and Armor and Craft Wondrous Item feats. The creator must labor for at least eight hours each day in a specially prepared laboratory or workroom. The chamber resembles an alchemist's laboratory and costs 500 gp to establish.
For stone constructs, the creator must be 16th level and able to cast either arcane or divine spells. Completing the ritual drains from the creator 100 XP for each of the construct's HD and requires geas/quest, limited wish, polymorph any object, and stone shape.
For metal constructs, the creator must be 16th level and able to cast arcane spells. Completing the ritual drains from the creator 150 XP for each of the construct's HD and requires geas/quest, limited wish, polymorph any object, and iron body.
When not working on the rituals, the creator must rest and can perform no other activities except eating, sleeping, and talking. If personally crafting the construct's body, the creator can perform the rituals while building it. If the creator misses a day of rituals, the process fails and must be started again. Money spent is lost, but XP spent are not. The construct's body can be reused, as can the laboratory.
Completing the ritual drains the appropriate XP from the creator and requires casting any spells on the final day. The creator need not cast the spells personally; they can come from outside sources, such as scrolls or other assisting casters.
