I made kind of what you were looking for along with an aspect of the end to try it with! The player defaults with 250 mana but I'll keep working on it and updating you. IDK what addons it needs just search up 'Skript actionbar addon'. Anyway here is the code (Its pretty simple)

Code (Text):

1. on join:
2.     set {mana.%player%} to 250
3.  
4. every 10 ticks:
5.     loop all players:
6.         send action bar "&b%{mana.%loop-player%}%&b/250" to loop-player
7.  
8. every 50 ticks:
9.     loop all players:
10.         if {mana.%loop-player%} is greater than 250:
11.             stop
12.         else:
13.             add 25 to {mana.%loop-player%}
14.  
15. on rightclick with diamond sword:
16.     name of player's tool is "&9Aspect of the End":
17.         if {mana.%player%} is greater than 49:
18.             add -50 to {mana.%player%}
19.             send "&aUsed &6Instant Transmission&a! &b(-50 Mana)" to player
20.             set {_aote.use.teleport.block} to block 8 in front of player
21.             teleport player above {_aote.use.teleport.block}
22.         else:
23.             send "&cYou don't have enough mana to do that!" to player

--- Double Post Merged, Mar 18, 2021, Original Post Date: Mar 18, 2021 ---

There are also a couple of bugs which is where sometimes your mana will go 25 mana above the limit.

 

Code (Text):

1. every 10 ticks:
2.     loop all players:
3.         send action bar "&b%{mana.%loop-player%}%&b/%{maxmana.%loop-player%}% Mana" to loop-player
4.  
5. every 20 ticks:
6.     loop all players:
7.         if {mana.%loop-player%} equal to {maxmana.%loop-player%}:
8.             stop
9.         else:
10.             add 10 to {mana.%loop-player%}
11.  
12. on rightclick with diamond sword:
13.     name of player's tool is "&9Aspect of the End":
14.         if {mana.%player%} is greater than 49:
15.             add -50 to {mana.%player%}
16.             send "&aUsed &6Instant Transmission&a! &b(-50 Mana)" to player
17.             set {_aote.use.teleport.block} to block 8 in front of player
18.             teleport player above {_aote.use.teleport.block}
19.         else:
20.             send "&cYou don't have enough mana to do that!" to player
21.  
22. on join:
23.     set {maxmana.%player%} to 100
24.  
25. command /manareset:
26.     trigger:
27.         set {mana.%player%} to 100
28.         set {maxmana.%player%} to 100

^ This might work

 

Code (Text):

1. every 2 ticks:
2.     loop all players:
3.         send action bar "&b%{mana.%loop-player%}%&b/%{maxmana.%loop-player%}% Mana" to loop-player
4.  
5. every 20 ticks:
6.     loop all players:
7.         if {mana.%loop-player%} is greater than {maxmana.%loop-player%}:
8.             set {mana.%loop-player%} to {maxmana.%loop-player%}
9.             stop
10.         else if {mana.%loop-player%} is smaller than {maxmana.%loop-player%}:
11.            
12.             add 5 to {mana.%loop-player%}
13.  
14. on rightclick with diamond sword:
15.     if {mana.%player%} is greater than 49:
16.         add -50 to {mana.%player%}
17.         send "&aUsed &6Instant Transmission&a! &b(-50 Mana)" to player
18.         set {_aote.use.teleport.block} to block 8 in front of player
19.         teleport player above {_aote.use.teleport.block}
20.     else:
21.         send "&cYou don't have enough mana to do that!" to player
22.  
23. on join:
24.     set {maxmana.%player%} to 100
25.  
26. command /manareset:
27.     trigger:
28.         set {mana.%player%} to 100
29.         set {maxmana.%player%} to 100

*Slightly modified.

The above code did not restore the Mana of other players when their Mana was at Max.
This may have worked fine in past versions, but it did not work correctly in the current version (minecraft 1.18.x skript 2.6.2), so I'm replying to a thread that is over a year old.
There may be a better way to write this, but I'm a newbie, so please forgive me.
Also, I'm using DeepL.