What’s the title of someone who practices Rune Magic?
If they’re adepts, they are called runemasters. Vitki or Vitkar are other titles for someone who uses the runes in sorcery.
The title used depends on what specific type of magick they practiced, and their gender. Below is a list of terms you will find in the lore to describe the various types of magicians.
seiðmaðr - seiðr-man
seiðskratti - evil-seið-sorcerer
seiðberendr - seið-carrier
spámaðr - prophecy-man
falsspámaðr - false prophecy-man
villuspámaðr - false prophecy-man
galdramaðr - galdr-man
galdrakarl - galdr-man
galdrasmiðr - galdr-smith
galdraraumr - great galdr-man
galdrameistari - galdr-master
galdradrengr - galdr-attendant
vitki - sorcerer
fjölkyngismaðr - sorcerer
fjölkyngisberendr - sorcery-bearer
gandrekr - gandr-man, gandr-warrior
kunáttumaðr - man who knows magic
vísendamaðr - man who knows
tauframaðr - charm-man
gerningamaðr - sorcerer
Völva - staff-bearer, seeress, sibyl
seiðkona - seið-woman
spákona - prophecy-woman
spákerling - old prophecy-woman
kveldriða - evening-rider
trollriða - troll-rider, rider of witchcraft
myrkriða - darkness-rider or night-rider
munnriða - mouth-rider
túnriða - fence-rider or roof-rider
kaldriða - cold-rider
þráðriða - thread-rider
galdrakona - galdr-woman
galdrakerling - old galdr-woman
galdrasnót - galdr-lady
galdrakind - galdr-creature
vitka - sorceress
fjölkyngiskona - sorceress
vísendakona - wise woman, woman who knows
heiðr - sorceress
fordcæða - evil witch
flagð(kona) - evil witch
fála - witch
gýgr - witch
hála - witch
skass - witch
These are found in the lore, but the modern term is hekser regardless of gender or practice. The choice is up to you.
Hope that helps a bit.