The Claighmor (one slot) is an example of a culturally specific weapon with additional bonuses for those with proficiency. A character may use it if they have hand-and-a-half swords (2 slots) as a proficiency, but only get the bonuses of the claighmor by having specific proficiency in claighmor. Other swords will often have bonuses for specific proficiency in them, Claighmor is given as an example of one such weapon:
Special Claighmor abilities:
Limited One-Handed Use: Claighmor may be used as a two-handed sword, or one-handed as a hand-and-a-half sword, but if wielded by a character with a STR of less than 18, the sword may only be wielded one-handedly for 5 consecutive rounds, and requires a CON check for each additional round it is wielded (you add a one point penalty to each CON check that follows, thus CON-2 on the 8th round [5 rounds, 6th: Con check, 7th: CON-1, 8th:CON -2]). Once a CON check is failed, the weapon cannot be wielded one-handed, the character is just too worn out.) Claighmor cannot be wielded with another weapon (or most shields).
Mounted use: As long as the character has the Claighmor and riding proficiency, he suffers no penalty from wielding it mounted.
Special Attacks: Claighmor can be used with any sword special attack that can be done with a large sword, and also: