Welcome to Pixelation! It looks very nice for a first pixel art.
The sword is moving at a linear velocity, try looking at the motion curves here:
http://easings.net/You could have it pull back slowly... then slam into the sword, for instance. Consider that when the sword hits the stone it might be touching the hole (that isn't presently drawn) and that will cause friction that will affect the velocity. If you do want the sword to actually enter the stone, consider just how much force that would require, or if you want the sword to have certain properties like heat that will cause the stone to melt, etc.
IMAGINE: A red hot magic blade hurtling towards the stone. It quivers up to the hilt after it strikes, and begins boring into the stone as lava dribbles from the edges of the sword. The molten rock lets off smoke as it cools and turns black.
d:o
And how is the sword moving? What's doing this, who? If it's a wizard, think about his magic, and what type of control he has over objects. Does he simply apply forces to objects? Does he influence the movement with jerky movements, or in flourishes? Is he using a wand, or his mind? Is it some celestial event? Invisible person? Esper? Is the sword sentient, and will it think about how to enter the rock? Once you decide on this, ideas for how to put it into the piece will come much easier.
The style rules you've created for yourself in this piece are clashing. the sword is outlined in black, but the rock is exempt from these rules. What a special rock! Usually you reserve breaking the style to convey specific things, such as motion blur. There doesn't seem to be a reason for having one outlined and the other not.
Also, whatever you're using to move the sword is allowing it to rest at uneven pixel coordinates and it's eating rows of pixels.
