Q:

Does this legendary action of Acererak's function this way?

Acererak in Tomb of Annihilation (Appendix D, p. 209) has this Legendary Action:

 Talisman of the Sphere (Costs 2 Actions). Acererak uses his talisman of the sphere to move the sphere of annihilation under his control up to 90 feet. 

In the description of the item,  it says:

 A creature whose space the sphere [of annihilation] enters must succeed on a DC 13 Dexterity saving throw or be touched by it, taking 4d10 force damage.  

It's unclear whether he can move it only in one direction, or whether he can change its direction as it moves.  If he can change its direction, that means he can move it through the same player's space 9 times, requiring 9 consecutive Dexterity saving throws.  
Is that correct?
For reference the description of the item is:

 [Talisman of the Sphere] When you make an Intelligence (Arcana) check to control a sphere of annihilation while you are holding this talisman, you double your proficiency bonus on the check. In addition, when you start your turn with control over a sphere of annihilation, you can use an action to levitate it 10 feet plus a number of additional feet equal to 10 × your Intelligence modifier.  (Acererak's Intelligence modifier is +8, so that's where the 90 feet comes from.)

Using Acererak's stat block, I gather that he can move it 90 feet, but the description indicates that he can levitate it x feet.  
Can Acererak only make it go up and down?  If so, how does he ever move it anywhere horizontally?  How did he bring it into the encounter? 

A:

The description of Sphere of Annihilation says:

The sphere is stationary until someone controls it. If you are within 60 feet of an uncontrolled sphere, you can use an action to make a DC 25 Intelligence (Arcana) check. On a success, the sphere levitates in one direction of your choice, up to a number of feet equal to 5 × your Intelligence modifier (minimum 5 feet).

Normally levitating is only vertical, this "direction of your choice" seems to mean it can levitate horizontally, or in other words hover.
It also says "in one direction" so it can't go back and forth or weave around friend or foe.
What is different with the legendary action or the talisman?
This seems ambigous to me.
The legendary action says 'moves the sphere up to 90ft.'. This alone could mean regular movement. However since it specifies the use of talisman, you'd check that description. In the talisman's description the "direction of your choice" part is missing leaving only the option to levitate normally.
Either

The legendary action removes the limit of "in one direction" causing it to be regular movement of 90ft for the sphere (as it only says 'move 90ft.)

or

The talisman increases the movement (other benefits aside) but removes "direction of your choice" to make it simple levitation.

In my opinion [How I'd rule this]

The legendary action description is just summary, pointing the end result of 10 × INT + 10 feet for this creature.

The talisman description is only referring to the sphere's original levitate ability instead of limiting it to only levitating vertically. Only making it easier to [take] control.

If this was the intended way the description should've been written as hovering instead of levitating.

