If you have the Forgotten Realms Player's Guide, the Spellscarred is an example of a multiclass-only class. Like normal multiclassing, you need to take a feat to gain access to the class and its powers, and once you've taken the multiclass feat, you can take Novice Power, Adept Power and Acolyte Power to swap one of your encounter, utility and daily powers with another one from the class.
As an example, let's say you have Frost Domain as a multiclass feat, and in order to take it, you need to have a divine power source and be a worshipper of the Raven Queen or whatever.
When you take the feat, you are considered multiclassed into the Frost Domain, and as a benefit (the domain power), say you get a +1 bonus to attack rolls for powers with the cold keyword.
When the character hits 4th level, he can take the Novice Power feat to swap one of his encounter powers with another encounter power from the Frost Domain power list. These could be original powers, or you could just pick existing powers to fill out the list.
Some existing powers that might work as frost domain encounter powers are: chill strike (wizard encounter 1), icy rays (wizard encounter 3), winter's wrath (wizard encounter 7), coldfire vortex, cold version only (warock encounter 13), and ice tomb (wizard encounter 17). Of course, you would need to modify the attack stat and implements of the powers so that they work better with a base divine character.
