Demon knight, Uncommon, 9g21r
Demon, Soldier
Attack: 25
Replace one of neighbouring cards with a Demon or Soldier from your deck.
A recurring issue in concept cards is where summoning gets very specific. Summoning by keyword is fine as the card pool is beyond the player's control. Summoning a particular rarity from your deck gives you more control but you have flat odds as there will be 15 from which to choose randomly.
When you design a card to summon a keyword from your deck, that gives the player the opportunity to stack the deck so the summoned card is not random. One could design a deck where the only card with the specified keyword is a low-cost Rare or any specific card with amazing synergy.
That being said, I'm not sure that would be an issue in this case. With both keywords, Demon knight is very likely to draw itself to the neighboring position. It's prohibitively expensive, so stacking the deck to draw a specific Soldier would probably result in having too few recruits to play it.
In general, I warn against combining keyword summoning with the "from your deck" restriction, but I just don't think the reasons apply in this case. So... carry on!