No, even though that deck uses all death cards, it doesn't contain at least 50% from any one of the 4 class groups. Here is an example of a (completely useless, so as not to give idea spoilers early) legal deck:
Hover over cards for details, click for permalink
52i 52j 52k 5fk 5fk 5fk 5id 5id 5id 5id 5id 5rm 5rm 5rm 5rm 5rm 74b 74b 74b 74b 74b 78q 7bu 7la 7oe 7qa 7ta 7ti 7um 81q 8pr
The deck has 15 cards (50%) from the "Monsters and Critters" class, 5 ice dragons, 5 devonian dragons, and 5 elite otyughs. The rest of the cards are all from death, darkness, gravity, fire, or time, or are pillars/pendulums of any type.