I’m going to answer “how do I combine Dervish with Lightning Maces,” rather than specifically how to get a slashing mace.
Aptitude Special Ability
The aptitude special ability from Tome of Battle can be applied to a weapon to cause feats that are specifically for another weapon to apply to the weapon with aptitude. It is likely that the designers meant just to let you switch Weapon Focus (dagger) to your aptitude longsword and similar, but the wording of the feat allows even feats where you never had a choice about the weapon type to apply to the aptitude weapon. This is frequently quite powerful, and occasionally completely nonsensical.
If you really want to use maces specifically, you could have an aptitude light mace, and then apply the Versatile Unarmed Strike feat to it. This is probably going to fall into the latter category for most groups, but it’s RAW-legal.
Alternatively, you could use an aptitude slashing weapon, ideally one with a large threat range (the kukri is almost certainly your best bet here: light slashing weapon with a large threat range), and then let Lightning Maces apply to it.
In either case, the Roundabout Kick feat works similarly to Lightning Maces, but for unarmed strikes: once again, aptitude can allow you to take the extra attack with your mace or kukri.
Combined with Disciple of Dispater, the kukris are looking at enormous threat ranges, which means you’ll score a critical on very-nearly every single attack that successfully hits. The maces are only somewhat smaller. Combined with Lightning Maces and Roundabout Kick, every critical triggers two attacks. Your number of attacks is thus more likely to increase rather than decrease; if you hit on the first two or three, you are statistically unlikely to stop attacking until the target is dead.
This is, of course, broken.
Were I your DM, I would allow you to have your slashing mace or allow Dervish to use non-slashing weapons without a second thought. On the other hand, Lightning Maces already is on my banlist. I also consider any combination of aptitude with a feat that couldn’t normally select the weapon in question to be something to be adjudicated by me, on a case-by-case basis.
The gods favoured weapon has the folowing influence on a cleric:
- A cleric gets proficiency with this type of weapon. I.e. if the cleric has quaterstaff as favoured weapon he can use any quaterstaff with no penalty.
Clerics are also proficient with the favored weapon of their deity.
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- The weapon summoned by a Spiritual Weapon spell takes the shape, critical threat and critical multiplier of the gods favoured weapon.
The weapon takes the shape of a weapon favored by your deity or a weapon with some spiritual significance or symbolism to you (see below) and has the same threat range and critical multipliers as a real weapon of its form.
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It grants no other benifits (if I'm missing nothing).
This means, you don't get the weapon for free and the weapon you buy is a normal weapon of its kind.
Best Answer
A deity's favored weapon is mostly flavor. Clerics can use whatever weapons they want, regardless of them being a favored weapon of their deity. There are a couple of mechanical effects for a deity's favored weapon though.
Clerics of the War domain receive Martial Weapon Proficiency and Weapon Focus for their deity's favored weapon if they do not already have it. This only applies to weapons that require Martial Weapon Proficiency. Simple weapons, such as a quarterstaff, are already in a Cleric's proficiency, and exotic weapons such as a claw bracer still require you take Exotic Weapon Proficiency. You still receive Weapon Focus as a bonus feat regardless of the nature of the weapon.
The spell spiritual weapon summons either your deity's favored weapon or a weapon with a spiritual connection to you. Spiritual weapons all do the same damage, but retain the threat range and critical multipliers of their shape.
The Clr spell weapon of the deity (Spell Compendium 237) grants the caster's weapon special abilities based on the deity the caster worships if the weapon the caster wields is the deity's favored weapon.
Favored weapons are used mostly as a roleplaying device. Warriors of a particular deity might be discouraged by their church from using weapons other than their deity's preferred weapon, for example. There is no mechanical penalty or benefit (other than the two listed) unless the DM creates one through storytelling.