If i take a Slann with, Unfathomable Presence (magic resistance 3), and Higher State Of Consciousness (only magical attacks have any effect) Does this make him immune to all attacks but magic, and magic saves of 1+?
It gives a 1+ Ward Save against all spells that allow Ward Saves. However, this does not translate to units or characters that have magical attacks from other means, such as magic weapons or innately magical attacks as is the case with Daemons of Chaos.
Never worth it to take the MR3 discipline on the Slann. Take the MR2 amultet for 30pts slann still has a 2+ (since a 1 always fails). I run him this way with the stubborn crown and he can hold up huge, non-magical attack units forever.