These are two words with a remarkable similarity in their pronunciation and spelling. The first of these, the one with the ‘m’, means something of immeasurable depth. This depth is generally not in physical terms but more in the figurative sense, and is used in the negative sense. Examples of its usage can be: abysmal ignorance/ abysmal concentration powers.
On the other hand, Abyssal is a technical term that means ‘Relating to ocean depths from 2000 to 5000 meters’. It’s a derivative of the word Abyss, and is used to refer to great or immeasurable depths.