After much investigation, and with some guidance or inspiration from the information you got from Marguerite, you manage to concoct what seems like a viable method to construct a necklace.

First, find a necklace of fine make such as silver or gold. An expensive metal will have sufficient socio-psychic resonance to be readily enchanted.

Pick an activation word that can trigger the attachment or detachment of the weapon from the necklace.

Engrave the hilt of the weapon and a sliver of metal attached to the necklace (or the necklace itself) with contrary sigils. The Sidhe sigils for “shrink” and “grow” should suffice for this.

Create a mixture of powdered lodestone, saltwater, and quicksilver. Polish both the necklace and the weapon with the mixture. Both should be still damp to the touch for the rest of the process.

Position a mirror adjacent to the necklace and the weapon, so that you can see the weapon clearly at full size next to the necklace, and so that you can also see it reduced in size next to the necklace via the mirror.

Anoint the mirror with a solution of bamboo ash, red wine, crushed vervain, and caster’s blood, to help bind the magic.

Light a candle and begin casting the spell of consolidation and binding that brings the elements together and makes them a single magical entity. The words of the casting are a request from elemental powers to enact the effect coupled with an expression of the will of the caster and informing the necklace and the weapon of the activation word. Continue the casting until the candle gutters.

The power level of the casting is 8. The effect is permanent and should also result in giving the necklace and the weapon added resilience. When part of the necklace the sword will shift in appearance slightly to appear to be more like an amulet.

If the casting is successful and uninterrupted, the activation word should now work when the weapon is held against the necklace.