I was having a conversation with Jacob Blackmon (by social-distancing-approved web interface), and he mentioned an idea of playing with how familiars and concentration spells in 5e interact. I found it interesting, and that developed into these Upgraded Familiars.

You gain an Upgraded Familiar with the find familiar spell, but must add additional material components as listed for each upgraded familiar below. Each uses the base stats of the listed normal familiar, but also has one of more special abilities.

Chameleon
Material: 50 gp worth of opal
Base stats: lizard
Special: A chameleon gives you access to the alter self spell, which you can cast using appropriate spell slots as if it was a spell you knew. The chameleon can take its action each turn to concentrate on an alter self spell you cast, which frees you from its limitations of a concentration spell.

Firefly, Giant
Material: 50 gp worth of amber
Base stats: owl
Special: A giant firefly can make as much light as a candle at will. It also gives you access to the dancing lights cantrip, which you can cast as if it was a cantrip you knew. The firefly can take its action each turn to concentrate on a dancing lights cantrip you cast, which frees you from its limitations of a concentration spell.

Mongoose
Material: 50 gp worth of pearl
Base stats: weasel
Special: A mongoose gives you access to the blur spell, which you can cast using appropriate spell slots as if it was a spell you knew. The mongoose can take its action each turn to concentrate on a blur spell you cast, which frees you from its limitations of a concentration spell.

Nightingale
Material: 50 gp worth of onyx
Base stats: bat
Special: A nightingale gives you access to the darkness spell, which you can cast using appropriate spell slots as if it was a spell you knew. The nightingale can take its action each turn to concentrate on a darkness spell you cast, which frees you from its limitations of a concentration spell.

Spider, Orb Weaver
Material: 50 gp worth of jade
Base stats: spider
Special: An orb weaver gives you access to the web spell, which you can cast using appropriate spell slots as if it was a spell you knew. The orb weaver can take its action each turn to concentrate on a web spell you cast, which frees you from its limitations of a concentration spell.

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