Weapon shapeshifters

Weapon shapeshifters are a type of spirit that most if not all of the Nine utilize but is especially popular with Zarthus. The Nine want to help their followers when they can, but they also want their followers to solve their own problems.
If the spirit views a mortal goal or quest as worthy, it may offer its assistance to the mortal and then transform into a weapon or tool.
The weapon or tool the spirit transforms into is an item of tremendous but it always has a limitation built in. A sword might work for a year and a day, or can only be unsheathed for three battles, 100 battles, etc. Once the term of service is up, the weapon or tool will dissolve and the spirit will return to its home plane.
Sometimes the spirit weapon will talk to the mortal, either audibly or telepathically and sometimes they are mute after taking weapon form.
Some spirits are rules sticklers and some are not. If the mortal wielding a spirit is obeying the word of the conditions set forth limiting the spirit's use but is clearly violating the spirit of the agreement, the spirit weapon may leave early. If the mortal exceeds the limits placed by the spirit but is clearly following the quest in spirit, they might stick around a little bit longer than agreed.
Given that spirit weapons are not enchanted in the same sense as most magical items, spirit weapons are usually immune to magical effects that nullify magical items. These magical weapons cannot broken down for reagents. If such a weapon is severely damaged, the weapon will dissolve and the spirit will return to its home plane.

Can they sire or bear half-spirit children?

Most say no. When they are taking the form of a weapon or item, most take the form of a talking cloud of mist or something similar. That why they are usually called "it" and not "he" or "she", but there are some accounts of shapeshifting weapons taking humanoid form albeit briefly.
There are unsubstantiated legends of the half-mortal children of living weapons that were conceived after the err celebration of the completion of the mortal's quest.
According to legends, the children of such unions are very handy with tools and weapons and frequently develop magical powers regarding the enchantment of magical items.