I like the idea behind Tower of Ther, but you should really consider some rework for the magus himself.
Simply put: If these cards are your first attempts at getting a feeling for balancing, basically your first dip into the new swimming pool, good ol' Ther just took you by the feet and threw you down a waterfall.
He's both: Totally overpowered and almost unplayable at the same time.
The problems:
- You need all five colors to play him, but his first ability stands at an overwhelming cost of FOUR white mana. For someone without a landbase consisting out of nothing else then duals, it's almost impossible to play him effectively before reaching around nine lands(!), this makes him extremely unreliable, aslong you don't have his legendary land.
- On the contrary, his second ability is just nuts. Countering any spell or ability on your creatures for one single mana is out of this world, especially since you don't have to tap him, allowing to spam this ability (ability, noteffect).
- Shadow is a very outdated ability; A more modern equivalent is fear, since intimidate would be hilarious on a five-colored creature. Also it's far too cheap with a cost of only two mana.
- With 4/2, he is far too vulnerable; Remember, that Hexproof only protects him from abilities/spells that explicitly target, so global damage or statreduction still kills him off very easily.
Some possible solutions:
How about setting equal costs for all of his abilities (like {b}{4} for black, {u}{4} for blue etc), so you can balance them out more easily? Instead of making him a disruptor, you could make him more of a supportive creature, where his abilities grant him a statbonus and an ability of the aligned color, that also effects all multicolored creatures.
For example
{w}{4}: Multicolored creatures you control gain +1/+1 and vigilance until end of turn
{r}{4}: Multicolored creatures you control gain +1/+1 and first strike until end of turn
{g}{4}: Multicolored creatures you control gain +2/+0 and trample until end of turn
{u}{4}: Multicolored creatures you control gain +1/+1 and hexproof until end of turn
{b}{4}: Multicolored creatures you control gain +2/-1 and Deathtouch until end of turn
Only problem would be the space on the card; WHO is supposed to read these small letters?^^ better make abilities with combined colors, like you did with the hexproof/shadow, but in a more refined way.
I like the concept of your idea, but it really needs some rework.