Slann are basically the be all and end all of magical lords as for around 500 points they can:
Get 2 free power dice when casting a spell.
Make a single enemy wizard within 24" discard all 6's when casting spells.
Know all spells from any of the 8 warhammer lords.
All enemies attacking them are at -1 to hit in combat.
Have a 2+ Ward save vs all ranged attacks.
Thats my favorite build for Slann using their current book.