DanmakuCore is distributed under a reasonably open license. You probably want to have a read through this if you want to do something with it other than just play.
You are completely free and have the right to Use, Share and Adapt the mod. These rights can not be removed from you as long as follow the license terms.
If you simply wish to play with the mod and do nothing else, go for it, this page is of no use to you.

General Clauses

Clauses that apply in general to any of the uses that follow.

Personal Permission Clause

I will not give personal permission unless I know the person asking or there's a really good reason for it, as giving personal permission implies endorsement. Asking for personal permission will likely just end up having you ignored or redirected back to this page.

Waive Clause

All restrictive clauses in this license can be ignored with personal permission. Read the above clause.

Extensive Clause

This license applies to DanmakuCore and all other code, assets or binaries found on the github repository unless otherwise indicated.

Informal Clause

This license will not hold up in court. I have no intention of suing anyone for breaking it, the worst that can happen to you if you do is that I'll get annoyed and ask you to rectify your mistake. If you use a public distribution platform (such as CurseForge) it's also possible your project will get taken down. Breaking the license also makes you a jerk, and you don't want to be a jerk, do you?

If you want to Distribute DanmakuCore

"Distribution" refers to making available binaries, assets, or source of the mod available from the original sources as part of your modpack or otherwise.

Attribution Clause

You must give appropriate credit to Katrix as the creator of DanmakuCore or the parts of it you're using. If you do any alterations the fact that you do so should also be indicated. A link back is optional but it would be cool if you would do so.

Non-Monetary Clause

You may not charge for access to the distribution itself or gain money through it, this includes any type of inline advertisement, such as url shorteners (adf.ly or otherwise) or ads in your service slowing the download down. This includes restricting any amount of access behind a paywall. Charging for ingame goods such as mod items or cosmetic features on a server does not count as distribution and falls purely under the Mojang Terms of Service.

If you want to Feature DanmakuCore

"Featuring" refers to using DanmakuCore in an environment where you do not distribute binaries, assets, or source. An example would be a YouTube Let's Play.

Thief Clause

You must not claim that you made DanmakuCore. Giving appropriate credit to Katrix as the creator of DanmakuCore makes you cooler, but you don't have to do it if you don't want to.

If you want to Use DanmakuCore Code or Assets

Usage of code or assets falls under the Extensive Clause.

Attribution Clause

Copyleft Clause

If you include DanmakuCore Code or Assets in your work (don't just depend on it), your project must be open source (have its source visible and allow for redistribution and modification) and include a clause similar to this one in its license.