Class idea help.

[Lanth Sor]

So i was thinking of building a assassin like class around the concept of increased crit multiplier instead of sneak attack die. To provide guaranteed crit while the target is unaware of the attacker or unaware they are an enemy, and maybe a heightened crit chance when the target is flat footed. Are there any major traps having a 95% crit chance or adding int to crit threat for flat foot would cause and issue?

[gooddragon1]

So i was thinking of building a assassin like class around the concept of increased crit multiplier instead of sneak attack die. To provide guaranteed crit while the target is unaware of the attacker or unaware they are an enemy, and maybe a heightened crit chance when the target is flat footed. Are there any major traps having a 95% crit chance or adding int to crit threat for flat foot would cause and issue?

The following types of monsters cannot be critically hit (exceptions in parentheses)

Undead - (Gravestrike Spell, Truedeath Weapon Crystal)
Constructs - (Demolition Weapon Crystal)
Plants - (Vinestrike Spell)
Elementals
Oozes
Swarm - Actually a subtype

There is a stance in Tome of Battle called Blood In The Water available at 1st level.

Roughly - For every critical hit you land +1 to attack rolls and damage rolls for 1 minute. I think further crits refresh the duration for all instances.

It is used in combination with the Lightning Mace feat.

Roughly - If you threaten a critical hit with a light mace, you may attack again.

Which is used in combination with the Aptitude Weapon Enhancement.

Roughly - +1 to attack rolls if you have weapon focus with this weapon and you may treat it as any other weapon for the purpose of feats.

Which is used in combination with threat range improvement modifiers to get the threat range of a kukri (18-20) to 2-20.

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Depending on whether or not you want to be able to "assassinate" anything, you could do the following:

abc ability (Ex)

If a ~ would threaten a critical hit against a creature, they may choose to forgo a critical hit and instead multiply the damage dealt from nonvariable sources by x?.

xyz ability (Ex)

Your threat range for the purpose of abc ability is improved by #. At # level this increases by # and every # levels thereafter. This does not affect the threat range of attacks for any other purposes.

This would make it different from a critical hit (since critical hits do multiply weapon damage dice, a Scythe doing 8d4 on a crit instead of 2d4) in a way similar to how lightning mace can affect things even though they are immune to critical hits. You can even chalk it up to an uncanny sense of how to hit more effectively (If you know what shatterpoint is from star wars, it's basically that without the force/magic) just like how a ranger with favored enemy or a fighter with weapon specialization is somehow hitting better against a target they can't see.