Knockdown is not a bad ability. as pointed out you are level 45. Also, I would suggest you put your charms on a mule and get a +Str charm for main slot for more damage (if you are using a sword, if using a dagger then go dex). Note: your charms are elemental charms and get more damage from Intelligence, you don't have much at all.
Just keep adding to that ability up to 40%, you will be around Master Quest by then and it will reset as stated by another after MQ and you are back to level 1.
Knockdown = Column, so if you see 2 mods on top of each other it is handy... also even vs 1 monster (Well or EP).
Note: if this is your first climb, you are set on default (2 normal monsters & 1 EP), so this ability is crap, however, once you complete your first MQ, the default settings will be disabled, thus 1-3 Normal & 1-3 Evil Presence can spawn.
Have fun and good luck on the next 26 levels to Master Quest... check your local MP for upgrades.
So dump the charms I have now ( sell on mp ) and get a charm or 2 charms with STR bonus for more sword damage.