I've made a homebrew creature called a Drachen (just German for dragon.) It's very similar to a Shadow dragon so I used an adult brass dragon template (for some reason D&DB didn't let you use Shadow dragon as a template.) I've tweaked some attacks, movement, languages, senses etc. not really making it any more or less powerful, just different.

However I want it to have the ability to phase in and out of the material plane and teleport when it does so, I added the following ability as a reaction, I'm still balancing it slightly but as it is do you think I should raise the challenge rating, at the moment it is still 13 to match the adult brass dragon template. 

"Shadow Form. As a reaction the Drachen can enter a void halfway between Shadowfell and the material plane. When entering the void the Drachen can teleport up to 30ft to any location it can see. Whilst within this void it is visible in both planes as a shadow of itself wreathed in deathly smoke, however it cannot be touched from either plane, it cannot be damaged by weapons or spells whilst in Shadow Form and all magical effects not projected by the Drachen itself will cease to affect it. The Drachen remains in Shadow Form for 1d4 rounds (including the round it took the reaction) and on turns spent in Shadow Form the Drachen can move but take no actions that reach into a plane of existence.

When the Drachen exits Shadow Form (always at the beginning of a round) it is allowed to use its missed actions for half (minimum 1) of the turns it spent in Shadow Form, and up to one full turn of movement."

As a reaction to what? 

An attack. I should put that.

It seems fine except for 5 things.

1) to quote Matthias_von_Schwarzwald, "As a reaction to what? 

2) There is no Recharge on the ability, so it could theoretically do it any and every turn... and why wouldn't you?  If it's a reaction to damage then it can only be attacked by a single PC a turn.

all magical effects not projected by the Drachen itself will cease to affect it.

3) This will feel SUPER bull*#@% to the players, especially casters.  It's next to impossible to get an ongoing effect on a Dragon between amazing saves and Legendary Save.  To have *any* negative effect dismissed with a ReAction is going to make any caster feel extra kicked in the groin.

The Drachen remains in Shadow Form for 1d4 rounds (including the round it took the reaction) and on turns spent in Shadow Form the Drachen can move but take no actions that reach into a plane of existence

4) This makes The Drachen have variable Time Stop.  It can move into the Shadow Plane, be unaffected by the PCs, buff itself, heal, whatever...  That's crazy.

When the Drachen exits Shadow Form (always at the beginning of a round) it is allowed to use its missed actions for half (minimum 1) of the turns it spent in Shadow Form, and up to one full turn of movement."

5) You don't say Round Up or Round Down, I'll assume Round Down.  This means after the Drachen gets to as a ReAction (so after it's normal actions) become incomporial, then do any buffs it wants to itself for 1d4 rounds.  It now gets free Actions.  With Round Up that's a 50% chance of 2 bonus actions and with Round Down that's a 25% of 2 free Actions.

I had actually addressed most of these issues already but D&D beyond seems to be not saving my edits sometimes randomly (yes I've been using the Save Changes button). This is the newer version, I've tweaked it a little bit, it actually has no buffs or healing that it can use so I removed the bit about magic from itself. I want this to be a good ability, which is why I considered upping the CR, it's main use would be to disengage from a fight reposition and reengage with a surge of energy as it were, I always assumed you round down unless stated so 25% chance of two free actions. I've got no problem upsetting casters who cant get an ongoing effect on this guy. They'll just have to change tactics and go for damage (seen as he has no healing)

Shadow Form (Recharge 5-6). As a reaction to an attack the Drachen can enter a void halfway between Shadowfell and the material plane. Whilst within this void it is visible in both planes as a shadow of itself wreathed in deathly smoke, however it cannot be harmed from either plane, it cannot be attacked by weapons or spells whilst in Shadow Form and all magical effects will cease to affect it (inclusive of the attack that caused the reaction). Inside the void, the Drachen can move but take no actions.

The Drachen can remain in Shadow Form for up to 1d4 rounds (including the round it took the reaction), the Drachen can exit Shadow Form early at any time at the end of another creatures turn, if not it is returned to its original plane at the beginning of the next round. When exiting Shadow Form, if it missed actions, it can use up one full turn of movement and half of its missed actions (minimum 1). 

So to summarise, this ability, nullifies one complete attack, cleanses any current magical debuffs, allows a free disengage, and grants 0-2 free turns.

Can the ability recharge while active? 

I hadn't decided but I don't think I'll go with no. That wouldn't seem fair.

Each use of this ability:

• Stops one attack by a PC
• Ends any spells that the PCs cast on the monster
• Lets the monster reposition itself without worrying about opportunity attacks
• Almost certainly recharges all the monster's recharging abilities
• Gives the monster an average of 1 free turn

If the monster just has this, Multiattack, and a breath weapon, calculate its average damage output as [its breath weapon damage (x2 because you should assume that it'll hit two targets)] + [its Multiattack damage].  In any case, this'll probably bump the thing's defensive CR up one notch per use.