Increasing the save DC of a death attack

[Darkness]

Things I can think of:

• More Int (via spell, item, etc.)
• Assassin's dagger (adds +1)
• Ability Focus feat (adds +2)

There's also an epic feat, but that's out of reach for most assassins.

So... How do I increase the save DC without taking more levels in assassin, similar classes or bloodlines (UA)? Are there any spells to this effect? Other useful feats? Magic items - a greater assassin's dagger, maybe?

 

[Storyteller01]

Would you accept a different rules set?

A different assassin, but the rules work:

When you study an opponent for 3 rounds, and they don't expect an attack, apply the following:

Double your sneak attack damage.
If you do over 50 points of damage roll for mass damage, with these changes: DC is 15 + int mod + 1 per 5 points of damage over 50.

You might house rule something from this.

 

[Darkness]

Hehe. I use some massive damage rules from UA that work in a similar way:

• Massive damage threshold is 25 + 2*HD
• Save DC is 15, +2 per 10 points of damage beyond your threshold
• If you fail the roll, you drop to -1d10 hp