Is it RaW that to use a bladed weapon 'sideways' to cause bludgeoning instead of slashing damage changes the weapon to an Improvised Weapon?
I ask because I wanted to cause bludgeoning against an undead IC as I spotted IC that the blade was doing little damage,,, OOC I said 'but the PC has been trained in this weapon'.. the GM said it was an Improvised weapon so I didn't argue just to keep the combat flowing..
We still killed that god damn abomination to life though ;)
RAW a slashing only weapon (that is a weapon not slashing and bludgeoning weapon such as the Dwarven Maulaxe) deals only slashing damage under it's proper usage. Your DM allowing you to use your weapon as an improvised weapon to deal bludgeoning damage is actually him being nice, because many people on this board argue that you can't even use a manufactured weapon as an improvised weapon.
Assuming your DM is nice and lets you use your manufactured weapon as a improvised weapon to deal bludgeoning damage you would treat it completely as an improvised weapon losing any magical enhancements, losing any feat benefits that apply only to a specific weapon, it would deal damage based on normal improvised weapon rules, you would take a -4 to attack rolls for using it as an improvised weapon, and will only score a critical hit on a natural 20 and will only have a x2 critical multiplier.
In short, yes. Your DM is right.
The answer to this may step outside of the actual rules, and into GM fiat. However, I will propose two "rules-light" ways to handle it.
1) Treat it as non-lethal damage. In early editiions of the game, before we had non-lethal damage, there were rules for knocking a creature out by literally using the fllat of the blade. Technically though, in our current rules, non-lethal damage doesn't change the damage type and... undead are immune to non-lethal damage.
2) Treat it as an improvised weapon. You will have a -4 to attack, and the GM choose it's qualities, which will probably be bludgeoning damage and probably a lower damage die. Also, feats and abilities specific to the sword you wield will not count, as you are not wielding a sword, you wield an improvised weapon.