Final answer:
The incantation 'Morsmorde' is used to conjure the Dark Mark, the sign of Lord Voldemort. It does not summon anything; rather, it creates the symbol in the sky and is closest to a Conjuration type of spell.
Step-by-step explanation:
The incantation for Morsmorde is, as you've mentioned, Morsmorde itself. This spell is not among the options you've listed like 'Avada Kedavra; Killing Curse' or 'Expelliarmus; Disarming Charm', or 'Accio; Summoning Charm'. The correct type of spell for Morsmorde is a Dark Mark Spell, which is used to conjure the Dark Mark, the symbol of Lord Voldemort and his Death Eaters. In the storyline of Harry Potter, it is often cast into the sky to mark the scene of a murder committed by the Death Eaters. It is not a type of summoning charm and does not fit neatly into the categories provided, but if one must choose, it is closest to a Conjuration as it brings forth the Dark Mark.