So I was working on a character build based on the idea of playing Venom/Carnage in Dnd, and I was kinda playing around with some class features to figure out what would be a somewhat optimized build so that I could play it in an upcoming one-shot. The build is Rune Knight Fighter 3/Lycan Blood Hunter 3+. The main features I was going for here were the two transformations that I can use- becoming Large with Rune Knight and turning into a monster (flavored to be an alien symbiote) with Order of the Lycan. Thing is, I pictured both transformations occurring simultaneously, and it feels somewhat awkward if I'm in symbiote mode but not large, or vice versa, so my question is: can you use a bonus action with either class feature such as this as an action? And, if RAW says no, would you make an exception for this circumstance?

The RAW answer is no -- this is an issue with a lot of MCs, as you can't rage and use Giant's Might on the same turn either, or use a Form of Dread, etc. etc. (Incidentally, Undead warlock's Form of Dread might be a good fit for this character idea too if you haven't looked at it -- scary appearance, temp HP and frightening creatures you hit in combat is pretty Venom-y)

The usual answer is just to pick one class' transformation and flavor it to be what you want cosmetically. You'll have to talk to your DM about allowing you to do one as a bonus action and another as an action if you want to stack them on the same turn 

If they say no and you still want to double up, you can always just flavor it to say the full transformation takes longer than one turn (six seconds), or you have a partially transformed stage sort of like a lycanthrope might

Quote from Chommeh_J >>

So I was working on a character build based on the idea of playing Venom/Carnage in Dnd, and I was kinda playing around with some class features to figure out what would be a somewhat optimized build so that I could play it in an upcoming one-shot. The build is Rune Knight Fighter 3/Lycan Blood Hunter 3+. The main features I was going for here were the two transformations that I can use- becoming Large with Rune Knight and turning into a monster (flavored to be an alien symbiote) with Order of the Lycan. Thing is, I pictured both transformations occurring simultaneously, and it feels somewhat awkward if I'm in symbiote mode but not large, or vice versa, so my question is: can you use a bonus action with either class feature such as this as an action? And, if RAW says no, would you make an exception for this circumstance?

5th edition have no allowance for taking your action as a lesser thing, with the exception of using your Action to Dash, which grants an extra instance of your movement speed.  To your question, RAW is no.

Would I allow it at my table?  Probably not, my table is pretty RAW in most instances.  Reason being is that one transformation, you would get:

• If you are smaller than Large, you become Large, along with anything you are wearing. If you lack the room to become Large, your size doesn't change.
• You have advantage on Strength checks and Strength saving throws.
• Once on each of your turns, one of your attacks with a weapon or an unarmed strike can deal an extra 1d6 damage to a target on a hit.
• You gain a +1 to melee damage rolls. This bonus increases by 1 at 11th and 18th level. You also have advantage on Strength checks and Strength saving throws.
• You have resistance to bludgeoning, piercing, and slashing damage from nonmagical attacks not made with silver weapons. While you are not wearing heavy armor, you gain a +1 bonus to your AC.
• You can apply your Crimson Rite feature to your unarmed strikes as a single weapon. You can use Dexterity instead of Strength for the attack and damage rolls of your unarmed strikes. When you use the Attack action with an unarmed strike, you can make one unarmed strike as a bonus action.

That's extremely powerful for one bonus action.

Sorry if my phrasing was unclear, what I meant was that I was asking if I could use one of these as a bonus action and the other as an action, meaning that the two parts of the transformation would take both my action and bonus action, and that would be my turn if I didn't want to trigger action surge as well. If you consider how much of these bonuses come from the Lycan, it is honestly less powerful to have to use an action to do both; since you already have the advantage on Str checks and saves, that means the same feature from Giant's might is neutralized, so you basically just have a Large size and extra 1d6 damage with unarmed strikes. This really is not that great mechanically, considering you're sacrificing two unarmed strikes that both do 1d6 anyway with just hybrid transformation because the combat would have to last at least 2 more rounds to make up that damage, so it's really just extra flavor if the DM goes with it.

You cannot use a bonus action as an action.

If I were DM, I'd allow it in this case, mostly because this seems more disadvantageous than to your benefit.

Your phrasing is fine, but 5th doesn't allow you to use your action to do bonus action things.  

Quote from AntonSirius >>

The RAW answer is no -- this is an issue with a lot of MCs, as you can't rage and use Giant's Might on the same turn either, or use a Form of Dread, etc. etc. (Incidentally, Undead warlock's Form of Dread might be a good fit for this character idea too if you haven't looked at it -- scary appearance, temp HP and frightening creatures you hit in combat is pretty Venom-y)

The usual answer is just to pick one class' transformation and flavor it to be what you want cosmetically. You'll have to talk to your DM about allowing you to do one as a bonus action and another as an action if you want to stack them on the same turn 

If they say no and you still want to double up, you can always just flavor it to say the full transformation takes longer than one turn (six seconds), or you have a partially transformed stage sort of like a lycanthrope might

 Thanks for the idea about the Undead warlock, I didn't know it had a transformation ability like that. If DM says no to using both on the same turn, I'll probably do that flavoring of half-transforming on one turn and then becoming fully transformed second turn of combat. The only issue with that is that Crimson Rite is ALSO a bonus action, so by the time my abilities all come online, the combat is going to be mostly over already. 

As others have said, No RAW you can not swap actions for bonus actions, bonus actions for actions, or any other combinations of things you can do on your turn. Things are only what they say they are, for better and for worse. You can consider this a flaw of the game or an intentional game design choice, but having the limitation on abilities like this is what the game is balanced around. You, as the player, need to make a choice on what you do during your turn. What choices you have access to are determined by your classes and race and those factors should be accounted for when you choose what classes/races you may want to play. Unlike in videogames or anime, Dnd is very heavily reliant on group efforts and is not very friendly to 'hold on I have to get powered up' sort of stuff. Frankly, this is one of the ways that action economy within a class is so important and is one of the reasons why I think the blood hunter is a terrible class. 

If you were at my table I would not allow this. If you want to get the benefit of multiple bonus actions, you need to spend multiple bonus actions. That being said, I would have no problem working with you to flavor something that gives you the general vibe you are working towards as long as it doesn't give you a mechanical benefit.